Steinbeck's characterization of Lennie in Of Mice and Men can be listed in several stages. 

1. In chapter one, Lennie is revealed to be like a little kid.  He is simple minded and cannot think for himself.  He also likes to pet things, even gross things like dead mice.  He will get away with whatever he can when he thinks he can, again like a child.  He doesn't mean to hurt anyone or anything, like the mouse, but he can't help himself.
2. In the clubhouse, his dependence on George is further revealed.  He is not allowed to speak because as soon as he opens his mouth he reveals his mental condition.  But he is a great worker.
3. His strength is confirmed by someone other than George.  He is, indeed, a great worker, as witnessed by others on the ranch.
4. His lack of intelligence and his strength come together and are highlighted by the fight with Curley.  Unskilled as a fighter, he merely wins the fight by catching Curley's fist in his hand and crushing it.  He first takes a nasty beating, however, because he will not fight back without permission from George.  Though Lennie gives Curley what he deserves, the kind of strength Lennie possesses paired with the mind of a child is foreshadowed as extremely dangerous.
5. Lennie identifies with Crooks as a fellow outsider, and he is a gentle and friendly man.  But he is also extremely innocent and naive.
6. Lennie is manipulated by Curley's wife into talking to her, which he tries to avoid because George has ordered him to.  Curley's wife, too, though, is starved for attention and manipulates Lennie into talking with her.  Lennie's lack of mental ability and great strength lead to his killing Curley's wife the same as he had killed the mouse.  There's no place for someone like Lennie in the novel's society, and Lennie is doomed. 

Steinbeck uses dialogue and character actions to reveal Lennie, one of the central characters in the novel.