In Of Mice of Men, author John Steinbeck very craftily creates two distinct voices for Lennie and George in order to develop their characterizations. Though they are close friends, they are very distinct characters. Lennie Small, contrary to his last name, is described as a very large man with a mental handicap. In contrast, George Milton is described as being small, with a very quick mind, and very shifty, sharp eyes.
Since Lennie is so large with limited mental abilities, he often doesn't know his own strength, a problem that leads to both the main conflicts and consequences in the book, including the fact that he unintentionally kills animals as he pets them and accidentally breaks a woman's neck. Since George is the smart one, he is continually looking out for his friend, reminding him of the farm with rabbits they'll one day have, removing dead mice from his hands, and taking on the responsibility of enacting a mercy kill to protect Lennie from an approaching lynch mob.
We can very clearly hear the differences between their voices in the very first pages of the novel, distinctions that help to characterize both their differing levels of intelligence and their roles within their friendship. For example, at one point, George, sitting by the pool of water, rants about how the bus driver dropped them off, saying the ranch they were headed to was "jes' a little stretch down the highway," leaving them to walk four miles in the hot sun when the driver could have taken them right to the gate. In contrast, Lennie doesn't have enough sense or understanding of what's going on to be angry. In greater contrast, Lennie's response to George's well-founded tirade is to say, "George? ... Where we goin', George? ... I forgot ... I tried not to forget. Honest to God I did, George."
Hence, even in this early passage, we can clearly see how Steinbeck is using differences in tone, words, and understanding to establish different voices for George and Lennie, and these differences help characterize their levels of intelligence and their roles in the friendship.