Steinbeck wrote Of Mice and Men with the intention of turning it into a stage play almost immediately. Both the New York play and the book came out in 1937. Steinbeck was thinking ahead while writing the novella. He would have to show Lennie all alone at the riverside campsite at the end. But in a staqe play there is no way to show what a person is thinking--except with the old-fashioned soliloquies such as Shakespeare used in Hamlet, which Steinbeck would not consider because they are not realistic--and his forte was realism. He thought of having Aunt Clara appear to Lennie as an hallucination, but he didn't want to give Aunt Clara a speaking role because that would destroy the impression that she was only an hallucination. The audience would think that this fat old lady had somehow magically found Lennie where he was hiding and that she had never really died after all. So Steinbeck explains in the novella that she exists only in Lennie's mind and that Lennie is supplying her dialogue. This is of minor importance in the book but of great importance in the stage play. Something has to be happening while Lennie is waiting for George--and Steinbeck did not want to open this scene with George already present. Steinbeck could have had Lennie talking to himself. After all, Lennie is mentally retarded and it would be credible for him to talk to himself. But this kind of talking could become tedious and even incoherent. In any case, it would not be dramatic. With Aunt Clara there is an intriguing dramatic confrontation. Steinbeck did not want the dramatic conflict to be between George and Lennie. There had already been enough of that. George has made up his mind to kill Lennie with the stolen Luger, and he has no desire to berate Lennie for killing Curley's wife in the barn.
And then from out of Lennie's head there came a little fat old woman. She wore thick bull's-eye glasses and she wore a huge gingham apron with pockets, and she was starched and clean. She stood in front of Lennie and put her hands on her hips, and she frowned disapprovingly at him.
And when she spoke, it was in Lennie's voice.
In Chapter Six, after Lennie has inadvertently killed Curley's wife, he retreats the the green pool of the Salinas River where the novel began. Lennie is thinking about how he has let George down again. Having been scolded by George each time he's made a mistake, Lennie remembers his Aunt Clara who helped to raise him and he imagines her scolding him just the same as George would. Lennie begins to consider living in the cave if George wants nothing more to do with him, and this is when he hallucinates about Aunt Clara, the "little fat old woman," as he imagines seeing her in front of him. Feeling guilty about letting George down, Lennie (in Aunt Clara's voice) starts to scold himself:
And when she spoke, it was in Lennie's voice. "I tol' you an' tol' you," she said. "I tol' you, 'Min' George because he's such a nice fella an' good to you.' But you don't never take no care. You do bad things."
Lennie answers her, in his own voice, and proceeds to have a conversation with himself: as Aunt Clara and himself. This is Aunt Clara's voice and influence but it is really Lennie criticizing himself.