John Steinbeck's novella Of Mice and Men is short, but it certainly gives a director enough material about the two primary characters, George and Lennie, to create an effective movie.
The story which happens from the time we fist meet the two men to the time when George feels he must shoot Lennie is quite gripping; it would be enhanced by the addition of the back-story. We are curious about how the two men came to be traveling together, just as the men in the bunkhouse are. They ask about it, and George says this:
“I ain't got no people. I seen the guys that go around on the ranches alone. That ain't no good. They don't have no fun. After a long time they get mean. They get wantin' to fight all the time. . . 'Course Lennie's a God damn nuisance most of the time, but you get used to goin' around with a guy an' you can't get rid of him.”
While that may be true, we know there must be something more than that to keep George from dumping Lennie somewhere, especially after he does the things he does that keep the men constantly on the move. So making the information about the promise George made Lennie's aunt a little more prominent would be useful to help the viewers understand both George's commitment to take care of Lennie but also his conflict at the end about how best to "save" Lennie.
We would also benefit from seeing a little more of what happened at the previous ranch, both as a foreshadowing and an appreciation for Lennie's true innocence about wanting to touch things that are soft. We see this first with a mouse, but how much more effective to see his attraction to the soft material of a dress, which of course sets off an entire series of events which will be repeated to an even greater degree on the next ranch. By the end of the story, we know that there is no saving Lennie from himself and can empathize with George as he realizes the same thing.
Finally, it would be interesting to watch the dream of the farm and rabbits evolve. It plays such a significant role in the story, but by the time we get it, it is almost a rote exercise. George
repeated his words rhythmically as though he had said them many times before. 'Guys like us, that work on ranches, are the loneliest guys in the world. They got no family. They don't belong no place. They come to a ranch an' work up a stake, and the first thing you know they're poundin' their tail on some other ranch. They ain't got nothing to look ahead to.”
This dream had to have grown, detail by detail, over time--or perhaps it was created as a full-blown proposition. It would be interesting to know which the director believes is true.
In all, there is plenty of character information in this short novella to develop a feature-length film. It has been done before, and it has also been produced for the stage.