In Of Mice and Men, Lennie is a static character. He does not develop.
In literature, a static character is a character that does not change. A dynamic character grows and changes in some way throughout the course of the story. Since Lennie does not change in any way, he is what we call a static character.
At the beginning, Lennie is a gentle giant who is not bright enough to understand that his strength can be deadly to small creatures. This is foreshadowed clearly by his dead mouse.
"Uh-uh. Jus' a dead mouse, George. I didn't kill it. Honest! I found it. I found it dead." (ch 1)
George does not really believe him, or doesn’t care. He knows that Lennie can be dangerous, even though he does not mean to. He reminds Lennie when they had to run after he tried to touch a girl’s dress because it was soft.
"Jus' wanted to feel that girl's dress- jus' wanted to pet it like it was a mouse- Well, how the hell did she know you jus' wanted to feel her dress? (ch 1)
George has to look out for and protect Lennie, because he cannot do it himself. He lives by what George tells him, and cannot survive on his own. The fact that Lennie does not change is demonstrated by the other incident when he did not let go, this time of Curley’s hand. George tells him to “get” Curley to avoid getting hurt in the fight, but Lennie does not know when to stop.
But Lennie watched in terror the flopping little man whom he held. (ch 3)
It is not long after that Lennie accidentally kills the puppy, and then Curley’s wife. Time after time he does not know his own strength, but he never changes. He can’t change. He does not have the mental capacity to change.