Foreshadowing is a key rhetorical device in Of Mice and Men. These elements of foreshadowing would fall under the rhetorical category of "logos" which is using logical ideas to convince the reader of something. In the case of foreshadowing, the idea is that if X happens, something similar to, or deriving from, X is likely to occur.
For example, when Candy finally agrees to allow Carlson to shoot his dog, he later regrets not doing so himself: "I ought to have shot that dog myself, George. I shouldn't ought to have let no stranger shoot my dog." This foreshadows the moment when George must choose whether to let Curley get to Lennie before he does. George chooses to do what Candy could not.
Steinbeck uses similes to compare Lennie to animals. In Chapter 1, Lennie gets a drink from the pool, "snorting into the water like a horse." When George tells Lennie to give him the dead mouse, Lennie is reluctant:
Slowly, like a terrier who doesn't want to bring a ball to its master, Lennie approached, drew back, approached again. George snapped his fingers sharply, and at the sound Lennie laid the mouse in his hand.
Lennie is also compared to a child, again using simile (a comparison of two things using "like" or "as"): "Sure he's jes' like a kid. There ain't no more harm in him than a kid neither, except he's so strong." The reader can form a picture of Lennie's personality from his dialogue and interaction but these descriptions help develop his characteristics for the reader.
The majority of the story is told from a relatively impersonal, third person narrator's perspective. The events are described as if they were seen by an objective observer who described the events with occasional insights into the character's minds. It is in the style of Realism. However, an unusual moment occurs when, in the last chapter, the narrator describes Lennie's hallucination, a departure from the Realist style. It is clear that Lennie is hallucinating because his hallucinations (Aunt Clara and the giant rabbit) speak in his voice. Lennie is essentially criticizing himself through these images. This is his usual routine and he knows George will do the same when they reunite. So, although Lennie does feel bad and is critical of himself, this routine is familiar and therefore it is comforting. In the past, George scolds him and they move on to another place.
When George appears, the hallucination disappears. "George came quietly out of the brush and the rabbit scuttled back into Lennie's brain."