It's amazing that a film as good as this one came from a small studio like Hal Roach. Sure, they released many of their films through MGM but they were tiny and specialized in short comedies with the likes of Laurel & Hardy, Charley Chase and The Little Rascals--but they were not known for dramas. Well, starting in the late 1930s, the studio tried their hand at such fare and in some cases did some lovely films--and "Of Mice and Men" is the best example of these new dramas.
As far as the story goes, it's much like the book and play---but with a few changes to meet the tough Production Code. The language was toned down--with swearing removed. Also, the film made a few minor changes in the play--but not many. What you see is essentially the Steinbeck story--and the studio trusted the source material enough to stick with it. They also should be applauded for picking two relative unknowns to star in the film--Burgess Meredith and Lon Chaney, Jr..
Overall, the film is a smashing success. The acting is very good, the direction also nice and the film kept me on the edge of my seat. The only negative, and it's very, very, very minor, is that a few of the scenes looked very much like they were filmed on a set. Still, it's one of the best films of the year and had it not come out in 1939 (the same year as "Gone With The Wind", "The Wizard of Oz", "Goodby Mr. Chips", "Mr. Smith Goes to Washington" and many other great films), it might have taken home some Oscars. Well worth seeing and a great example of a film made very well on a relatively small budget.