Look at the way both Lennie and George are first described. How is this initial description fitting when we find out more about each man?
George is first described as "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." George's restless eyes are especially important because they indicate he is constantly watching, watching out for the safety of Lennie and also watching for opportunities in life. It is George who insists on spending the night in a clearing so he and Lennie can be free for one night. George is also the one with the dream of the farm. His strong hands indicate he will be able to take care of Lennie when the time comes.
Lennie, on the other hand, is described in the opening scene, as dragging his feet "the way a bear drags his paws." This points to his great physical strength and his lack of intelligence. This also mirrors the description of him in the final chapter of the book when he enters the brush "as silently as a creeping bear." Like a bear, Lennie exhibits animal-like behavior that appear instinctual. Since instinct is something he cannot control, his is not capable of living in human society.