There's a fundamental decision that should be made early in the design of any IF: does the player character (PC) have a personality of his or her own, or not? By that, I mean not simply that the PC has a name and a background, though that should be a start; the Zork model, where the PC has no discernible persona, is widely viewed as inadequate. The question is really whether the PC has ideas and goals that are distinct from those that the player brought into the game. Since, ordinarily, the goals that the player brings in are quite simple, and look something like "see all the text and surmount all the obstacles," it might seem that even the most bare-bones PC will have his or her own objectives.
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In practice, though, that's not the case. More often, in classic IF, the PC's motivations supply an initial premise, or perhaps give the player an overall goal (one that, perhaps, explains why the Magic McGuffin acquired at the end has such fascination for the PC)--and at that point the author shoves the player into the game and commands him to start solving puzzles. What follows, in most cases, can be viewed as driven by the PC's motivations only if the PC is a cross between a magpie and a Games Magazine subscriber; most puzzle-driven games depend so much on the player picking up everything that isn't nailed down, and viewing every locked door as a personal affront, that the only thought that could conceivably animate the PC throughout is "you never know." In a small but growing minority of games, however, the entire course of the story, and everything the player does, can be tied directly to the PC's motivations. This is, I argue, an encouraging trend: many of the most interesting experiments in IF have come about because the author took the trouble to convey, and encourage the player to understand and embrace (for purposes of the game), the motivations of the PC. Further development of that trend, one might logically conjecture, could produce even more interesting experiments.
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I. A Brief History of PC Development
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A. Infocom
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(This section contains major spoilers for Beyond Zork, Planetfall and Plundered Hearts, and minor spoilers for Lurking Horror.)
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What I have described above as the old paradigm of PC characterization will be readily recognized by most IF veterans, but some examples from the days of Infocom may illustrate the point. In Zork I, the PC has no name, no distinguishing characteristics, and, seemingly, no interest in anything but treasure. (Even that is debatable; nothing in the game indicates that your character is genuinely excited by all this treasure-hunting. It could plausibly be argued that your character secretly longs only for a nice snack, or a good nap, and that the heartless PC is dragging him/her around the map against his/her will. The point is that there isn't much evidence either way; the game never stops to say "You tremble with avarice at the sight of the jeweled trident" or any such thing.) In Beyond Zork, the PC has an initial goal--finding the Coconut of Quendor. (For the uninitiated: yes, really.) The player is told initially that the Coconut of Quendor resides with the Implementors (the Infocom authors themselves, who regularly made cameo appearances in their games), so the PC's initial goal is presumably to find the Implementors, but in practice the puzzles that the player solves to reach that point have no foreseeable connection to that goal. (They include such things as solving a riddle written on the side of a cliff and searching a cellar for a wine bottle in order to obtain a giant onion.) When the player finds the Implementors, the Coconut is snatched away by a nefarious presence, and the goal becomes finding that nefarious presence. (In fact, the Implementors explicitly state as much, lest the player think the game is over.) The same process follows: the player solves a long string of puzzles, and for only a few of them (the last few) could it be said that solving the puzzles has a foreseeable connection to tracking down the enemy. The PC's motivation, therefore, serves only as a framing device--or, less charitably, an excuse for the puzzles.
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In fact, that was true for most of Infocom's games, though to varying degrees. The fantasy and science fiction games were more recognizably a set of puzzles with little direct relation to the overall objective, except perhaps a thematic relation (most notably in the cases of Trinity and Zork III). Gathering a set of objects--whether treasures or some sort of recipe for a desired concoction--was particularly popular, since it allowed the PC to indulge his/her magpie impulse, picking up everything in sight, and to regard every barrier as a challenge on the theory that you never know where those objects might hide.
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Planetfall is a particularly striking example of Infocom's general inattention to motivation, as the game's premise (you escape a doomed space station and crash-land on a seemingly deserted planet) seems to suggest a variety of actions (calling for help, say, or finding a way off the planet)--and the game suggests, through various red herrings, that those obvious actions are possible when in fact they are not. The real goal, suffice it to say, is wholly unguessable, and the protagonist arguably seeks out the climactic location only because some monsters chase him/her there. Several of Infocom's other games--particularly the mysteries--had puzzles that were more closely related to the plot, but even in those the PC had little personality of note; if it could be assumed that the PC was just as interested in solving the mystery as the player was, there was no need at any point for the player to take stock of the PC's motivations. Representative in ths respect is Lurking Horror, where the PC, a university student, is given a clear initial motivation, namely finishing a paper; when he discovers that something has gone wrong with his paper and that he might learn more in a certain department, the motivation becomes getting to that department. So far, so good, but various sinister events happen on the way (and more happen when he gets there), and at some undefined point the PC apparently says "forget the paper--I'm going to figure out what's going on around  here!" That's not entirely impossible, but it does essentially merge the PC with the player, since both apparently have as a goal Understanding Everything In Sight--which conveniently allows the PC to do all sorts of stuff that merely finishing a paper wouldn't justify.
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One notable exception to the general rule, however, was Plundered Hearts, where the PC is a 17th-century girl charged with the tasks of (a) surviving various dangers, (b) rescuing her father from the clutches of an unsavory fellow, (c) defending her own chastity from the wiles of the same fellow, and (d) catching the eye of, rescuing from the same villain, and ultimately marrying a dashing young swain. All of the goals can ultimately be accomplished, naturally, though it's sometimes unclear which one the game wants the player to focus on, and it's possible to bog down if the player focuses, say, too exclusively on saving the PC's skin and insufficiently on bailing the father or the swain out of jams. (Interestingly, multiple endings are possible, and the suboptimal ones correspond roughly to achieving only some of the motivations: in one, the heroine saves herself but abandons her lover and father; in another, she sacrifices herself to save her lover; and in another, she saves her lover but not her father.) Still, having too many motivations represents a refreshing change from having no particular motivation (or having a general goal that has no obvious impact on any particular decision), and the puzzle-solving in Plundered Hearts is usually sufficiently plot-driven that it cannot fairly be reduced to "go poke around." For example, at one point the plot  depends on your gaining admission to a ball, which requires (a) producing your invitation and (b) getting appropriately attired. It's not obvious that any of the overall motivations for the game require attendance at the ball, but the game helpfully supplies you with some--"you haven't been to a ball in months!" A few stretches of the game involve simple exploration--but it's exploration of
quite limited environments, and for clearly defined objectives (getting off a drifting ship in one case, finding your father in another).
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That moment, though, is an exception; the game is relatively restrained about pointing you in the direction of the story it wants to tell (and hence spelling out the PC's motivations). At one key point, for example, you take a moment to signal your friends that you need help (even though your need is not, at that point, all that pressing), and your friends conveniently show up just in time later; it's possible, as I recall, to simply not think to signal, with the result that the friends don't show up later and things go awry. There, for purposes of keeping the story on track, the game might simply have said something like "Before you do anything else, you should signal your friends," a
bit intrusive but helpful for purposes of making sure the PC and the player are on the same general page. It helps that Plundered Hearts isn't a particularly long game and the story isn't all
that wildly complex; the player might conceivably do the signaling simply because the opportunity presents itself. A more complicated storyline might demand more such signals about what
the PC really wants.
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It's also worth noting that the Plundered Hearts PC's personality is considerably more vivid than that of any other Infocom PC; there are things, for example, that you won't do because it's not
in your nature, or because you've been advised against those things. Whether that strong personality is a cause or effect of the overabundance of motivations--or is wholly unrelated--doesn't ultimately matter.
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B. Latter-Day IF
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(This section contains major spoilers for Delusions and minor spoilers for Jigsaw, Sunset Over Savannah, Little Blue Men, and Varicella.)
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Plundered Hearts stood as an exception to the general pattern for quite a while, even after commercial IF died out, and it was not until the latter half of the '90s, when IF authors began to move away from puzzle-solving as IF's raison d'etre, that PC motivation once more became the subject of experiments. An early precursor to that trend came in 1995 with Graham Nelson's Jigsaw, a time-travel game that moves the player through a variety of 20th-century scenes, each time chasing a nemesis determined to change history and each time attempting to set history back on course. The larger question of motivation--why is the PC so convinced that history must unfold just as it did?--is not explored as fully as it might be, but everything that the PC needs to do in each self-contained scene proceeds reasonably logically from the objective--which is itself obvious (if the player knows the history that needs to be set right).
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Similarly cognizant of the problem of PC motivation was C.E. Forman's Delusions in 1996, where the PC's goals change repeatedly over the course of the game: the initial goal is to test a VR simulation, a subsequent goal is to escape a different simulation, and a final goal involves defeating a certain villain. The transitions between goals are reasonably logical, enough so that that there's never a question about what objective the player is pursuing at any given time. For example, the player emerges from one simulation into a suddenly deserted lab (which turns out to be another simulation), and immediately both player and PC have a new goal: to figure out where everyone went, and, when the nature of the problem emerges, to find a way out. When the way out is found, the PC stumbles directly into a conversation involving the villain, and the new motivation is obvious. Perhaps more importantly, however, a fourth goal emerges along the way, namely that the PC wants an explanation of some nagging inconsistencies tugging at the edge of his consciousness. The subtlety with which this particular goal emerges--through various discoveries that initially appear unconnected--makes Delusions noteworthy, as the game manages to encourage the player to solve the problem without spelling things out directly.
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Other recent games have likewise tackled the problem of motivation in innovative ways. In Michael Gentry's Little Blue Men, for instance, the PC's primary goal is to eliminate things that annoy him--in the game's parlance, to become "frosty" rather than "steamed"--and pursuing that goal single-mindedly (how to do that is usually, though not invariably, clear) sends the story in all sorts of unexpected places. Achieving the PC's motivations means, in other words, that the player has to take on a distinctly antisocial mindset--meaning, if not an outright conflict between the player and the PC, at least a bit of tension. The execution isn't, alas, quite perfect; there are moments where the player's sheer curiosity has to take over from the PC's quest to maintain his emotional balance, as the author can't quite connect the PC's goals with certain bits of puzzle-solving. Ivan Cockrum's Sunset Over Savannah is in some ways the obverse of Little Blue Men: the PC has decided that she hates her job and is trying to decide whether to quit, and wanders a beach, encountering various experiences that remind her of her forgotten joie de vivre. The PC's sole motivation is to make the decision; the player's motivation becomes seeking out the experiences that will help the PC decide. Again, the puzzle-solving strains the model; the player repeatedly compels the PC to do things that simple restlessness would be unlikely to motivate (since a few of them are a bit on the suicidal side). Still, the underlying motivations are unusual, which is worth
noting in itself, and they keep the player thinking.
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Adam Cadre's Varicella, like Little Blue Men, poses the problem of a PC with unattractive motivations; unlike Little Blue Men, though, the story is sufficiently well engineered that the PC's motivations consistently drive the plot. The PC is a palace minister aspiring to a recently vacated throne, and scheming to bump off those with precedence in the succession order. The game's puzzles all tie directly into one of the various murders--there are no occasions where the PC has to get through a locked door or solve a Towers of Hanoi puzzle in order to obtain some unforeseeable object that will prove useful. (In other words, the author didn't just happen to pick a good plot; the
game is designed in such a way that the player's magpie/Mensa member personality rarely, if ever, shows through.) Other games of note in this regard include Stephen Granade's Losing Your Grip, in which the PC goes through a lengthy internal odyssey, confronting various inner demons--and while it would be difficult to say that the PC ever has an overall objective, at various times he is given smaller goals that advance the plot in a variety of logical ways. (There are a few exceptions--scenes that seem more akin to a set of puzzles than a stage in the PC's odyssey--and, not surprisingly, those scenes were heavily criticized.)
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II. PC Motivations and Game Design
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(This section contains major spoilers for Spider and Web, FailSafe, The Weapon, LASH, and 9:05, and minor spoilers for Enchanter, Sorcerer, My Angel and Photopia.)
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What are the implications of all this for the would-be designer? First, shorter is usually better; most of the games that have done a creditable job of accounting for the PC's motivations have been relatively short. Most IF puzzles are set-pieces--it isn't easy to develop a lot of puzzles that fit seamlessly into a game, unless your story is "solve the puzzles in your latest Games Magazine." The longer the game, and the more puzzles, the more likely it is that the PC will need to solve puzzles that he/she has no obvious reason to solve.
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Second, many of the games that have developed strong PC motivations have been segmented, so that a particular motivation, once accomplished through solving a puzzle or two, gives rise to another motivation that is accompanied by another puzzle. (Of course, if your game's so short that there's just one puzzle--and everything the PC does has an obvious connection to that puzzle--segmenting is less of a big deal. Enlightenment, by Taro Ogawa, is one example.) Each goal should be relatively immediate, sufficiently so that puzzle-solving has an obvious connection--if there's a door to unlock, there should be some good reason for thinking that something interesting to the PC is on the other side of the door. Many games begin this way, typically with some humdrum task--in Lurking Horror it was writing a paper, in Wishbringer it was delivering a letter--but lose motivational focus when the wide middle section of the game comes along. The solution may lie in escaping the clutches of the short, narrow introduction, long, wide middlegame, and short, narrow endgame structure, as it's the middlegame that tends to be closest to an unabashed collection of puzzles (whereas the humdrum-task introduction and the my-objective-is-before-me endgame tend to have goals that are directly related to the puzzle-solving). Enchanter and Sorcerer, where the beginning tasks are, respectively, getting into the bad guy's castle and getting out of your own Guild Hall (and the endgame objectives are essentially kill-the-bad-guy), are examples. Obviously, the goals also need to be less cosmic than save-the-world (which Infocom was rather fond of but the latter-day IF community has moved away from).
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Third, imputing emotions and thoughts may be necessary to the tell the story you want to tell, if that story is at all complex. Delusions did this to great effect, in part because it was unexpected; a PC without strong reactions to most stimuli had a progressively stronger reaction as it confronted the inconsistency, and it helped convince the player that this was an avenue worth exploring. To be sure, it's possible to "discover" your own feelings--you could, say, have the PC read a letter that
he or she has written--but if you're already inside the PC's head, it seems easiest simply to tell the player what's going on in that head, absent some compelling reason not to share that information with the player (as discussed below), Again, the puzzle-oriented roots of IF tend to encourage an emphasis on the objective--but the current state of the art calls for more subjectivity, a less Hemingwayesque persona at the center of it all. The occasionally confusing nature of Plundered Hearts, which, as noted, generally avoided informing the player of the PC's thoughts, suggests that less, in this regard, is not necessarily more.
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Fourth, emphasizing story and deemphasizing puzzles may make it easier to tie the PC's motivations to the course of the game (assuming the story is even vaguely plausible). One of the more striking examples of this was Jon Ingold's My Angel, where there are very few puzzles (and what puzzles there are tend to be small-scale and short-term) but the game tells a reasonably complex story. The PC's immediate motivations are clear throughout the story, even though he doesn't really have a discernible large-scale objective (and there are good reasons for that); the story begins with him following his companion, and each stage of the journey brings up a new objective. There, overcoming whatever obstacles impede the journey tells the story in itself, and there's little ambiguity about why the PC continues the journey--and hence the few puzzles feel adequately motivated. (In theory, they could have fit into the story even if they'd been more numerous and more conventional puzzles, but then you'd get questions about why exactly this journey seems to lead through so many barriers and require so much riddle-solving. Again, the idea is a _plausible_ story.) Conversely, it's hard to tell a convincing story if the character at the center of it remains a cipher.
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Illustrative in this respect is Adam Cadre's Photopia, which alternates between fantasy and real-life--and most of the fantasy segments have puzzles associated with them, while the real-life scenes have interaction but no puzzles. Each of the scenes from real life features a different PC, each of which has a reasonably obvious motivation in that scene, and each scene advances the plot in measurable ways. The fantasy bits, on the other hand, have thematic connections to the plot but don't (by the very nature of the story) advance the ball, and the puzzle-solving in each of those segments is a mite aimless--even when you know what you're trying to do, there's no obvious way to go about doing it. There are good reasons in the context of this particular game why the path of the fantasy story would wander, but the contrast is still striking: the progress of the story hinges on clearly motivated non-puzzle interactions on the one side, and on less clearly motivated puzzle-solving on the other. 
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More attention to the PC's motivations not only tends to go along with better storytelling, moreover, but also tends to make for better puzzles, for those that still prefer to write puzzle-oriented games. Set pieces, or "soup can" puzzles (so named when Russ Bryan commented in a newsgroup discussion of the graphic adventure Seventh Guest, "what the hell kind of villain thwarts the hero's progress with soup cans in the kitchen pantry?"), have become increasingly disfavored for that very reason: they don't fit into the plot, and the PC (usually) has no particular reason to solve them. Likewise, players often complain about puzzles that require suicidal tendencies (as in Sunset Over Savannah) on the theory that the PC should be assumed to be a relatively rational actor. It's worth remembering that several of the most innovative puzzles of recent years have turned on well-developed PC motivations. In Andrew Plotkin's Spider and Web, for instance, the PC is a captured spy who lies to his interrogator about his movements inside the lab where he is caught. The player, however, does not know that the PC is lying, and hence has no reason to believe that the PC has any reason to conceal anything from the interpreter--and the puzzle that calls for the player to draw that conclusion is a masterstroke. This sort of split between player and PC is difficult to accomplish unless the PC has his/her own goals and motivations, and
centering the story on the PC motivations, in this case, makes both the story and the puzzle work. PC motivations, and their concealment from the player, play similar roles in Jon Ingold's FailSafe and Sean Barrett's The Weapon, and in each case the game's primary puzzle involved picking up on those motivations.
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Furthermore, some of the most interesting IF experiments of recent years--experiments that have pushed the bounds of IF theory--depended on developing the PC's persona and motivations. Spider and Web, FailSafe, and The Weapon are all examples; Paul O'Brian's LASH, where the PC is a robot that eventually rebels against the humans controlling it (and who, naturally, control it in much the way a player controls a PC), is another. (One can read into this device a bit of commentary about how the conventional IF PC resembles an automaton mindlessly picking up objects.) One of the cleverest such experiments is Adam Cadre's 9:05, where the PC is a burglar who has broken into a house and killed the occupant--but the player, waking up the next morning in the house, assumes it to be his and takes the PC off to the victim's workplace in the victim's car (and promptly gets caught). The joke there, obviously, is that the PC has motivations that he doesn't get around to sharing with the player, and that the player's assumption that the PC has certain motivations (getting to work) turns out to be unwarranted. Many interesting things have yet to be done with the player-PC relationship, and giving the PC a persona--with personality, worldview, and highly specific motivations--open up the possibilities.
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III. Conclusion
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Character development in IF--whether of the PC or of the non-player characters (NPCs)--has long lagged behind puzzle design and other aspects of world-modeling, in part, no doubt, because of the difficulty of coding even vaguely realistic human interactions. While development of a PC doesn't depend on coding as such, it's arguably true that a game with minimal human interaction (as IF has historically tended to be) tends to need less in the way of PC development; who cares about crafting an intricate PC psychology when the PC has nothing to match wits with? (Technically, of course, the human interactions could be offscreen, or appear in cut scenes, but the tendency, I think,
has been simply to write barren, austere games.) NPC interaction has matured over the past few years, however; the hybrid conversaton structure (accommodating both menus and general topic selection) that Emily Short has pioneered and that some others have attempted allows for conversation with considerably more depth and range. That, in turn, calls for a more developed PC to hold up an end of those conversations. The newfound emphasis on the storytelling aspects of IF--partly driving and partly resulting from advancements in NPC developement--likewise calls for a PC whose complexity matches that of the narrative. Letting the PC's motivations, rather than the player's motivations, determine the course of the story is among the challenges that IF developers should look to meet.