Since Lennie is all alone and not doing much moving because he is in hiding from the lynch mob, the only way to show his thoughts and feelings was by resorting to a sort of stream-of-consciousness technique. This was a challenge to the author because he is exploring the consciousness of a man who is feeble-minded. In addition to visualizing scenes from his past, Lennie talks aloud to himself, to his Aunt Clara, to George, and to a large imaginary rabbit, all of whom talk to him in his imagination. This brief scene calls to mind the section in William Faulkner's novel The Sound and the Fury (published in 1929) in which the reader is taken inside the stream-of-consciousness of the idiot named Benjy.
Lennie hallucinates and imagines the figure of his Aunt Claire and he also imagines a gigantic rabbit that mocks him for ever believing that he could tend the rabbits.
Here the methods of characterization being used are thoughts and feelings - because his hallutionation of Aunt Claire makes him feel sorry for his actions.