The movie version of the story was released in late 1939 and received favorable reviews. It was mostly faithful to the book, with two notable exceptions. One was that the greater freedom of the motion picture medium allowed the filmmakers to "open up" the story, as they generally do with adaptations. Steinbeck had written the novella with the intention of adapting it into a stage play, which he did in 1937, the same year the book was published. In order to make the adaptation to the stage simple, the author had confined the action to two sets, a bunkhouse and a barn. The only exception was the campsite by the pool in the first and last chapters. This could easily be represented on a stage with a fake campfire and electric lights. Everything that happens outdoors in the novella is only described or represented with sound effects. For instance, Steinbeck describes the clang of horseshoes and the stomping of horses hooves. When Carlson shoots Candy's dog, the men in the bunkhouse hear the sound of one shot outside.
In the movie version, there are many panoramic outdoor scenes showing the beautiful California farmland, the big wagons being pulled by teams of horses, and the crews of men working hard at harvesting barley. Steinbeck would undoubtedly have included such scenes in his novella if he hadn't been thinking of turning his story into a stage play.
The other notable difference between the book and the movie is with Curley's wife. In the book the wife is described as a "kid" and as "jailbait." She is obviously only a teenager. Steinbeck made her very young to justify her irresponsible behavior and the fact that Lennie could kill her so easily by shaking her and breaking her neck. We picture her as slender and frail, not fully mature, probably as young as sixteen. In the movie version the role was played by Betty Field, who was born in 1913 and would have been twenty-six when the picture was made. Her character is a hard-boiled, worldly wise woman who should have known better than to be flirtatious with Lennie, the mentally limited giant who crushed her husband's hand to a pulp and has just killed a puppy by petting it too hard.
Steinbeck was a relatively obscure regional writer before Of Mice and Men. He had a spectacular and unanticipated success with that story. It was a successful novel, a successful stage play, and a successful movie. The book has now become an American classic.