Steinbeck intended to write about the harsh lives of the itinerant farm workers in California, a subject he knew about from personal experience. Whenever a writer has to deal with a number of characters of the same sex and social station, he has a problem of differentiating them enough so that the reader can tell them apart. Carlson's chief distinguishing feature is that he is surly and unsociable. He is one of the older men. He owns a German Luger, which becomes important at the end. He is not friendly with the other men because he doesn't want to accept the fact that he is one of them. He takes his bitterness out on Candy's dog as well as on poor Candy. Slim explains to George that most of the bindlestiffs are loners. Perhaps many of them are like Carlson in not wanting to accept reality, in not wanting to accept their present existence as their final fate. This might explain why they are always on the move: they are hoping to find a better life somewhere else. As Slim tells George:
You know how the hands are, they just come in and get their bunk and work a month, and then they quit and go out alone. Never seem to give a damn about nobody.
They are mostly uneducated and semi-literate. They can't see the bigger picture--that they are only little units in a vast army of unskilled and unwanted millions of bedraggled, downtrodden, homeless men wandering blindly all over America. These are the "mice" referred to in Steinbeck's title. His book and the play into which it was made were startling at the time because they gave the readers and audiences a glimpse into a world they scarcely knew existed.