I assume you are referring to the 1992 adaptation starring John Malkovich. The major difference between this movie and the novel is that the filmmakers "opened up" the play by showing the vast fields of California with the men and horses working in the sun. This was entirely missing in Steinbeck's 1937 book, apparently because he intended to adapte it into a stage play, which he did. Please refer to the Introduction in the eNotes Study Guide for Of Mice and Men as well as to the media adaptations via the reference links below.
Steinbeck's original novel or novella is very short, consisting of only six chapters. All the settings are indoors except for the opening scene at a riverside campsite and the closing scene at the same place. Most of the play takes place in the bunkhouse, in the barn, or in Crooks' room adjacent to the barn. It seems obvious that Steinbeck was thinking about making it easy to adapt the story for the stage while writing the novella. There is an unusual amount of dialogue and little prose exposition. All important information could be communicated to the theater audience by what the characters say to one another, so the work of adaptation would be easy for Steinbeck and his collaborator George Kaufman.
The 1992 film version of the novella makes up for the absence of the panoramic outdoor scenes which seem to be called for in a story about farm workers. The book mentions horses but only represents them by sounds of stamping hooves and jingling harnesses. The movie shows the big teams of horses which were still in use for pulling agricultural implements in the 1930s but which have since been replaced by all kinds of specialized farm machinery.
Another difference between the novella and the motion picture is to be seen in Curley's wife. Steinbeck makes a great point of her extreme youth in the book. Judging from her revelations to Lennie in the barn, she is only about sixteen years old. But in both the 1939 and the 1992 film versions Curley's wife is played by a much older woman. In the novella several of the characters, including George, refer to the girl with such terms as "the kid" and "jailbait."
Steinbeck probably wanted Curley's wife to be very young because of the reckless way she flirts with the other men, the injudicious way in which she approaches Lennie alone in the barn and gets herself killed, and the apparent fragility of her adolescent body which made it more plausible that she could have her neck broken by being shaken. Curley obviously has a big inferiority complex, which makes it seem plausible that he would choose to marry such a young girl and that he would have such a hard time relating with her. The dreams of glory she expresses to Lennie are those of an immature girl, not a grown woman married to a rancher.
Curley's wife lay with a half-covering of yellow hay. And the meanness and the plannings and the discontent an the ache for attention were all gone from her face. She was very pretty and simple, and her face was sweet and young.
Both the 1939 adaptation starring Lon Chaney Jr. and Burgess Meredith and the 1992 adaptation starring John Malkovich use Steinbeck's original dialogue without significant changes. The novella is easy to adapt to stage or screen because Steinbeck planned it that way. Steinbeck apparently chose to make one of the characters retarded so that his friend has to explain everything to him in detail--and be explaining the plot to the audience at the same time.
Besides what I have written in the above answer, I would like to add a bit of interesting trivia. In the 1939 version of the story, the character of Curley is played by an actor named Bob Steele. This character frequently appears riding on a beautiful horse and then galloping away. Obviously an expert rider, Bob Steele played in a long list of cowboy films going all the way back to 1930, when he appeared in Oklahoma Cyclone. A viewer who watches the 1939 version of the film on DVD may feel that Bob Steele looks familiar. It turns out that he played the part of Lash Canino in the 1946 film version of The Big Sleep, which starred Humphrey Bogart and Lauren Bacall. Canino was the heavy who killed Harry Jones in Walgreen's office by giving him a glass of whiskey laced with cyanide and was subsequently shot to death by Bogie at Art Huck's garage in Rialito.