The introduction of Lennie presents the reader with the depiction of a man of limited mental properties, one who is animalistic and reacts in a more instinctive, rather than cerebral, manner when faced with threatening situations. In Chapter One, for instance, Lennie lumbers behind George Milton, dragging his feet with his arms hung loosely at his sides. After they make camp in the clearing, George is angered by Lennie's insistence on ketchup for his beans when he has been informed that they have none, Lennie responds in a less than rational manner, childishly asking George if he wants him to go away, suggesting that he could live in a cave.
Another irrational response that Lennie has occurs when he is frightened. Again, in a more animalistic manner, Lennie has a surge of adrenaline when frightened; therefore, he becomes even stronger than he is naturally. Also, he reacts against that which threatens him in the defensive mechanism of an animal: he tries to overcome that threat. In the episode of Weed, when frightened by Lennie, the girl "squawks and squawks." Fearing that someone will come and hurt him, George tells Slim that Lennie became "so scared all he can think to do is jus' hold on....He was so scairt he couldn't let go of that dress."
In another instance, after the pugnacious Curley steps into the bunkhouse, threatening the men, George makes the comment, "...this Curley punk is gonna get hurt if he messes around with Lennie." Of course, Curley is, indeed, harmed when he lashes out at Lennie for laughing and George tells Lennie he can fight back. Lennie's herculean strength smashes Curley's hand. And, similarly to the incident in Weed, Lennie fears that Curley's wife will summon others, so he tries to keep her quiet, but accidentally breaks her neck.
When Lennie is frightened he has the tendency to react quickly without hesitation. Since Lennie is mentally inhibited he is not fully aware of the power he is capable of neither the harm he does. However Lennie is really good at listening to George especially when George tells Lennie to stay away Curly's wife. But this does not work out Curley's wife entices Lennie into petting her hair but she starts to panic and shout at how rough Lennie is petting her. This causes Lennie to get scared because he doesn't want attract people and George ends up finding out that he disobeyed him. So in trying to makes Curly's wife be quiet Lennie breaks her neck.