In the first few paragraphs, the narrator describes the physical characteristics of George and Lennie. These physical traits are similar to their personalities. Therefore, Lennie and George are opposites in physical stature, demeanor, and in terms of their personalities.
The first man was small and quick, dark of face, with restless eyes and sharp, strong features. Every part of him was defined: small, strong hands, slender arms, a thin and bony nose. Behind him walked his opposite, a huge man, shapeless of face, with large, pale eyes, and wide, sloping shoulders; and he walked heavily, dragging his feet a little, the way a bear drags his paws.
As they begin to converse, it becomes quickly apparent that George is the leader of the duo and Lennie is the follower. In fact, their relationship is quite similar to a parent-child relationship with George, of course, being the parent. Lennie lets his mind wander, often to the dream of having a farm and the rabbits. George shares this dream but his mind is always conscious of his environment. When Lennie is daydreaming about the rabbits, George is thinking about the next job and how to keep Lennie in line. In times of crisis or confrontation, Lennie panics and George keeps his head. They are opposites in almost every way except for the fact that they are both good people who look out for each other.