I think that it is part of Steinbeck's genius that he depicts Lennie through the conversation between Slim and George. Steinbeck describes this discussion as almost one between a priest and penitent, and in the process, it becomes evident that George has wanted to explain this relationship for some time. Lennie is shown to be a man- child figure. His stature and strength belies the fact that is a child, with the same sensibilities and outlook as a child. Lennie's character is also revealed to be one that is not often in control of his strength, an element that shows his greatest strength to be a weakness. The discussion of what happened to George and Lennie in Weed is something that will haunt both of them in their experiences on the ranch. Slim listens, and as he listens, so do we as more insight is gained into both Lennie's background, the role of Aunt Clara making George promise to look after him, as well as how Lennie is a full time responsibility. At the same time, George reflects how Lennie's loyalty is unquestioned, such as the time when he jumped in the water when George told him to as a joke. In these small tidbits of flashbacks, Steinbeck uses the conversation to help develop an empathetic construction of Lennie's character.