In Steinbeck's Of Mice and Men, Lennie Small is a big, strong, slow-witted man who travels around the country with his friend George. For the most part, Lennie is obedient to whatever George tells him to do. Earlier in the work, George had told Lennie that "If there’s any fightin’, Lennie, you keep out of it.” Lennie readily agreed with this command.
Accordingly, in Chapter 3, when Curley attacks Lennie because he thinks Lenny is laughing at him, "Lennie’s hands remained at his sides; he was too frightened to defend himself." So, it would appear that a combination of fear and remembrance of George's earlier command were in Lennie's mind.
Lennie allows Curley to pummel him until finally George gives Lennie the order to defend himself. It is only after Lennie hears George's command to fight back that Lennie defends himself by catching Curley's hand in his own and breaking just about every bone in his hand.
Early in John Steinbeck's Of Mice and Men, the reader is introduced to the story's two protagonists, the somewhat diminutive but smart and capable George, and the mentally disabled giant of a man named Lennie. The two men are walking and it becomes clear from their conversation that they are headed in the direction of a ranch where they will seek employment. It also becomes clear that something bad involving Lennie happened at a previous place of employment. As George is instructing Lennie on how to behave when they reach the new ranch, he admonishes his partner, "An' you ain't gonna do no bad things like you done in Weed, neither."
In Weed, it will be revealed, Lennie inadvertently frightened a woman whose dress he enjoyed touching, causing an incident that forced the two men to flee the town. Knowing of Lennie's inability to control his actions and formidable strength, George instructs Lennie to simply stand still and to say nothing while he, George, addresses the foreman of the ranch where they hope to find work. Unfortunately, the ranch owner's son, Curley, is a hotheaded bully whose beautiful wife serves as a constant temptation to the ranch hands. In no time, Curley causes trouble for Lennie when he fails to understand that this new employee is different from the other ranch hands:
His eyes passed over the new men and he stopped. He glanced coldly at George and then at Lennie. His arms gradually bent at the elbows and his hands closed into fists. He stiffened and went into a slight crouch. His glance was at once calculating and pugnacious. Lennie squirmed under the look and shifted his feet nervously. Curley stepped gingerly close to him. "You the new guys the old man was waitin' for?"
"We just come in," said George.
"Let the big guy talk."
Lennie twisted with embarrassment.
Knowing from his conversation with other ranch hands that an altercation involving Curley is very possible, George has instructed Lenny to refrain from getting into any fights, as to do so would almost certainly result in serious injury or death for the other party. Curley is small but tough and loves to pick fights with larger men. George can smell trouble and tries to set his friend straight:
"Look, Lennie! This here ain't no setup. I'm scared. You gonna have trouble with that Curley guy. I seen that kind before. He was kinda feelin' you out. He figures he's got you scared and he's gonna take a sock at you the first chance he gets."
When Curley does eventually attack Lennie, convinced in his jealous rage that Lennie was laughing at him, the bigger, stronger man can do nothing but stand and absorb the punches until George rescinds his command and Lennie inevitably blunts Curley's next blow and breaks the latter's hand.
Lennie had been instructed by George to avoid getting into a fight with Curley. When the latter begins to pummel his friend mercilessly, however, George changes his mind and yells at Lennie to "get him." The result, as noted, is a temporarily defeated Curley now suffering from a crushed hand.