Steinbeck wrote Of Mice and Men with the intention of turning the book into a stage play almost immediately. He called his book a "playable novel," meaning that it could easily be adapted to stage or screen. The book is unusual in that it contains the barest minimum of exposition or description. The characters talk to one another and their dialogue conveys most necessary information to the reader, as is the case of most stage plays and motion pictures. It was essential to the story that the reader, or future theater audience, should know about what had happened in the town of Weed just before the story opens. Ordinarily a novelist might simply narrate the incident or recreate it as a flashback in time; but in Steinbeck's book it is interesting to note how the Weed incident is narrated by George both to Lennie and later to Slim in the form of narration. So when Steinbeck turned the book into a stage play the actor playing George could do the same thing.
Lennie makes a good "foil" for George in both the book and the stage play, since George is required to explain everything to this retarded friend who has difficulty in understanding and in remembering. While George is explaining past incidents, present plans, and future hopes to Lennie, the reader (or future theater audience) gets all the information that is needed to understand the story.
It happened that Steinbeck was exceptionally gifted at writing dialogue, which may explain why he decided to make his story into a "playable novel." He has been ranked along with Ernest Hemingway and Dashiell Hammett as one of the best dialogue writers America has ever produced. Good dialogue conveys information unobtrusively and characterizes the speaker at the same time. Some of Steinbeck's really remarkable dialogue is contained in his masterpiece The Grapes of Wrath. All of the major characters in that novel are "Okies," who migrate to California in their jalopies piled high with their kids and their meager possessions during the Depression Era. The dialogue spoken by these uneducated sharecroppers is probably the best thing The Grapes of Wrath has to offer.