In order to understand the structure of this short novel, it is essential to know that Steinbeck planned to turn it into a play, as is explained in the Introduction in the eNotes Study Guide, which can be accessed by clicking on the reference link below. The play was produced in New York in 1937, the same year the book was published. The book is unusual in that it relies so heavily on dialogue. This is obviously intended to make the adaptation to a stage play simple, fast, and faithful to the original. All the important exposition is conveyed through dialogue, as it is in a play. The play must have practically written itself.
Steinbeck wanted to show Lennie all alone by the river at the place where George had presciently told him to hide if he got into any trouble. The author knew that he couldn't just show Lennie sitting there by himself, so he invented some hallucinations which would allow for dialogue. On the stage the images Lennie is supposedly seeing in his imagination would actually appear. For example, the first hallucinatory image is that of Lennie's Aunt Clara, who is described as a little fat old woman wearing thick eyeglasses and a huge gingham apron. An actress would be used for this brief scene, but Steinbeck significantly specifies that "when she spoke, it was in Lennie's voice." On the stage this would show that the woman is an hallucination.
The next hallucination is a gigantic rabbit. No doubt the stage version would include a big stuffed rabbit, which would not present any great problem. Again Steinbeck specifies that "it spoke in Lennie's voice too."
These hallucinations and the accompanying dialogue fill up the time that Lennie is alone. They also show Lennie's thoughts and feelings. When George finally appears, "the rabbit scuttled back into Lennie's brain." In the play the big stuffed rabbit would be jerked offstage by an attached cord.
Throughout the novella it can be seen how Steinbeck was thinking of what his story would look like on the stage. He confines his sets to only two, partially for the sake of economy. Everything takes place in the bunkhouse or in the barn. Crooks' room is just a part of the barn and would probably not be represented as a separate set on the stage. The opening and closing scenes by the river could be done on stage with a painted backdrop. Steinbeck does not even label the six sections of the book as "chapters," because they are intended to be "acts" in the adaptation. Anything that happens outdoors is either not shown or represented by sound effects, including the shot that kills Candy's dog.