Well sorry for the delay, I really wanted to get this up sooner, but resources have not been on my side. 

In American Literature there are few novels quite as celebrated or acclaimed as John Steinbeck's Grapes of Wrath. A landmark in 20th Century fiction, as well as one of the most well read and controversial books of its time. Following the success of Steinbeck's earlier Of Mice and Men (1939), it seemed a no brainer that this Pulitzer winning follow up would find its way onto the screen. Hollywood took no shortage of time to get it out there, and before the book was even a year old the film was already done. 20th Century Fox won the bidding war, and John Ford, quickly emerging as the studios top director was given the assignment. The man sent to adapt it however was Nunally Johnson, not Dudley Nichols who was Ford's main screenwriter during this period. Johnson stuck as close to the book as he could, and perhaps closer than any other Hollywood picture of the time. 
The world of film however was behind most of the other arts at this time, typically because of the large cost/risk factor involved. Film always took a little longer to get up to speed, and thanks to the Production Code there were some things that could be printed in books but not shown on film. Johnson kept a lot of the dialogue the same, but there's a separate language in the book that couldn't be spoken. With the uproar caused by Gable saying "damn" the year before it's no wonder why phrases like "sons of bitches" and "you go to hell" were left out. You can't slight the screenwriter for this, but it nevertheless makes you wonder how the film would have sounded if certain restrictions were not in place. 
Hollywood film, particularly of the studio era, has a fluidity that makes one not instantly pick up on details. There can be holes, flaws, inconsistencies that smooth cutting and good acting can make you ignore. Many times it takes repeated viewings just to spot these let alone be bothered by them. The Grapes of Wrath is not without flaws, inaccuracies, and holes. It would be nitpicking to mention them, so let me nitpick. For starters Tom Joad (Henry Fonda) has an older brother Noah. Now reading the novel I didn't really remember the character in the movie. When Tom is reunited with his family (which was also handled differently) however Noah is mentioned and shown. His whopping dialogue consists of "Hi Tom". Noah doesn't really appear throughout the rest of the film, and suddenly the family is picking peaches in California and there are only 4 men. They do mention and deal with Connie's abandonment, but why did they just ignore Noah running off, and never even mentioning it? Seems like a scene or two was cut here, but this could have been fixed with a line of dialogue, its as if Mr. Johnson forgot that the character was even in the movie. 
The whole end structure was turned around, and somewhat unnecessarily. The scenes near the end of the movie were in the book, even Ma Joad's (Jane Darwell) "We're the people" speech, but damned if they weren't in a completely different order. In the book the Joads got to the first Hooverville (which isn't mentioned by name here) just as they did in the film. There was a fight with a cop, Casy (John Carradine) was arrested, and Connie ran off, this much was accurate. Then the film takes a different stance. In the book the Joads relocated to the government camp right after, here they go right to strikebreaking and picking peaches. True Casy was reunited with Tom there, and killed, but there was a distance between it. This has it looking as though it was the very next day, and why would Casy have become a labor leader in such a short time? If the time of them being in camp, and Casy actually being imprisoned, which is where he found his calling, were in the film then this scene would have its proper context. It does work, but it worked better in the novel. Tom being found out and being forced to leave his family was much different and later in the book. Here Tom is found out by a cop walking through the camp checking license plates, which doesn't make a great deal of sense, in the book however it is Ruthie bragging to another kid accidentally that gives him away. Tom was also in hiding at this point, because of his face, whereas in the film despite the scar he is out in the open and even briefly working. 
Now the film has another vital difference that can be considered a benefit or a fault depending on whom you ask. The film is a lot more positive and optimistic. The Joads don't starve here, and only once do they really allude to how little food they have. Tom finds work as do they all, and they're leaving to get 20 days of work. There is no desperation, there is no starving, and there isn't the community of the novel. There was a camaraderie of the poor in the novel thats all but done away with here, very little is kept, with the exception of the scene in the diner and Ma giving the camp kids stew. Which could be attributed to growing animosity towards leftist sentiments. Also the film doesn't deal with what happened to Rose of Sharon's baby. The movie makes Tom the main character, and for awhile he was in the book, but the book became Ma Joad's story eventually as she became the head of the family. Pa is admittedly not the head in the film, but it doesn't imply that Ma took over, instead it is more Tom whose considered the leader of the bunch. 
Now a lot of things have to do with time. Perhaps they couldn't show everything, and there wasn't time to show the Wilsons, or Al and Tom's knowledge of cars. There wasn't time to show how prospectors and car dealers took advantage of the Okie's desperation. Again this is a negative point, and the film wants us to leave somewhat happy, feeling that the Joads are gonna pull through, whereas the book leaves you wondering whether they'll survive the winter. That said the movie could have been a little longer and not gone overboard, and much could have been added, straightened up, and elaborated upon, but would the film be considered as memorable? 
The acting in this film isn't so much a host of great performances as much as it is iconography. These people are all symbolic. The Joads have to represent all of displaced sharecroppers from the Dust Bowl. Muley Graves is all of the farmers refusing to leave his land. Casy is all the spiritually minded leaders and red agitators. Tom is everyone who stood up, and had to hide out. Ma is every head of the family struggling to keep her flock together. One cop in a camp has to stand for all the dirty police men determined to keep migrant workers out. 
At the most recent screening of this film someone mentioned how timely it is. Ive always thought of it as a story of and about the depression and how it affected a certain group and we should all be thankful it's not like that anymore. However someone mentioned that Mexicans are todays Okies. Its true, they go to California, they take the dirty jobs no one wants, they also take any and everything they can get for it, and they're also feared by the land owners, because nothing is more frightening than a starving man. As the gas station clerks mention, the Okies aren't quite human, and its true they survive because that's what they have to do, and this is why they're hunted and persecuted, because if they are allowed to get a little piece of land, they'll get a bigger piece of land, and so on. 
Henry Fonda and Jane Darwell give the best performances of their careers here. Fonda was by this point a Ford regular and reading the book there isn't anyone else you can picture as Tom, it is perfectly inspired casting. Fonda goes all out in the role too. He doesn't push for tears or sentiment, but he's tough, respectable, and his face can say more than any words can, and that's part of the beauty of the novel as well. Hell Fonda even sings, in a horrible out of key voice, but that's certainly going above and beyond. Darwell won an Oscar for her work here and few people would argue with it. Ma Joad is a tough woman and it takes a certain kind to pull it off. Somehow you get the sense early on when Ma and Tom first are reunited that these two actually could be mother and son. Three years later they would be reunited in William Wellman's The Ox-Bow Incident (1943), and Darwell was far from a kind mother figure in that. 
John Carradine was a man that needed direction. Left to hack scripts and B movies he could be laughably bad. In the hands of a man like Ford however he could be expertly used. He is Casy, you feel it, his speech, his actions, even the bony, haunted, angular face is perfect for it. You get the feeling that when he says something it's worth paying attention to, and you respect him just as the Joads do, even if he may be slightly off center. Carradine unfortunately despite making a lot of films was never as good as he could have been. Just as he was emerging as a great actor, he seemed to take the wrong turn and the rest of his career, despite a few more Ford appearances, was all downhill. 
This film holds a special place among Ford's work as being one of only two films to feature Greg Toland as the director of photography. It is also said that the work of Toland on this film is what brought him to the attention of Orson Welles. Toland was given a much freer reign shooting The Long Voyage Home (1940), but here his camera work is very much another character in the story. Everything is in focus, and so much is shrouded in shadows. Look at the scene where Tom and Casy go to the abandoned Joad home. Tom lights a candle, and in typical Hollywood fashion that would light up the room as if someone turned on a 100 Watt light bulb, here it is still a candle light and everything else is still in shadow. It was a remarkable accomplishment to photograph with so little light, and helped to set up the dominant postwar style of noir. There is no real explanation why Toland and Ford didn't make more films together, but it was most likely studio related, although Ford did keep a very consistent crew throughout his career. 
The film on the other hand is a typical Ford picture. It is ironic that the great director of Westerns won all four of his Oscars directing films in every genre but the one he was most famous for. Grapes of Wrath however is a Western. Sure it isn't about cowboys and Indians, but it is still of the same vein. The passage out west could just have easily been a family traveling to San Francisco in the great gold rush, or some homesteaders making their way across the Oregon trail. Fonda's Joad is just as much a western hero as his Wyatt Earp in My Darling Clementine. The folks still have square dances, like to drink, get in brawls for good fun, all hallmarks of Fords westerns. Even the brief appearance of Ward Bond is enough to remind one that it is a Ford film, and his cop could just as easily be the sheriff from the Wagon Master (1950). So when people say this film is contrary to Ford's other work, you may want to look a little closer. 
Alfred Newman was a resident Fox composer, and worked on pretty much every studio film for them, just as Franz Waxman worked for Paramount and Max Steiner for Warners. His score is typical of many of the era, but there is a theme to the film, and thats "The Red River Valley". The song pops up at the dance, is played in the opening credits, and the orchestration features variations on it throughout, including Tom's final departure and that last shot of him silhouetted on the hilltop. The music is memorable, and whenever you hear that standard, chances are you'll recall this film, and that's what great movies and great movie music can do. 
The ending is a matter of controversy, and not just because of how it differs from the book. The legend is that Ford wanted to end the film with Tom's departure and that final shot of him on the hill which bears a great deal of similarity to Ingmar Bergman's last shot in The Seventh Seal (1957). The studio apparently wanted a more uplifting ending, and therefore they threw in Ma's speech. The speech is corny and almost ruins the entire movie, but it is in the book. It just came at an earlier stage, when Tom was still with the family, and their spirit was very much still intact. I think it is used poorly here, and the ending that Ford supposedly shot and preferred few would debate as being the better of the two. On my own however, I would stop the film after Tom's departure, and consider that the final shot, it makes the film have a much more poignant conclusion, one to even rival the novels. The ending in the film is one that reminds you that not only is this a film, but its one from Hollywood, and one trying to manipulate you, or one that apologizing to you for being so depressing for 126 minutes.