In Bradbury's Fahrenheit 451, Clarisse McClellan introduces the concept of original thought to Guy Montag. As the story begins, the reader meets Guy Montag, a man who takes everything at face value.
When Montag meets Clarisse, she asks him to question his world. She reminds him of nature, something he has forgotten to notice:
"There's dew on the grass in the morning."
He suddenly couldn't remember if had known this or not...
She asks him if he ever reads the books he burns. His response is that it would be illegal to do so, but he learns that Clarisse's family does illegal things all the time: things we would laugh at. Her uncle, for instance, got in trouble for being a pedestrian one time.
Then Clarisse asks Montag a simple question—and it drastically changes things for Montag because he learns that he has no idea how he really feels. She asks him if he is happy. He thinks it is a ridiculous question...until he goes home and thinks about it: he realizes he isn't happy at all.
In instances like these, Clarisse awakens the thinker in Montag: the person that never asks questions of himself or society, but is manipulated like almost everyone he knows...except Clarisse, and eventually Faber. Clarisse's questions make Montag question his job as a fireman. He burns houses without ever thinking of the lives he destroys in the process. Until now, it had been an impersonal action on his part:
You weren't hurting anyone, you were hurting things!
However, when he goes to burn the house at 11 No. Elm, and the owner is inside, refuses to leave, and ultimately sets her house and herself on fire, it can no longer be impersonal for Montag, for he now thinks about everything. He questions why people do what they do, and he decides he can no longer be led mindlessly through his life.
Early on, a speeding car kills Clarisse, but not before she convinces Montag to think about how he lives his life. Her questions galvanize his character forward to take ultimately make his own choices as to how he will live his life. Standing up to Beatty and the Mechanical Hound symbolize his rejection of the government's laws. Clarisse opened his eyes to things he had forgotten and things he had never noticed. From her questions, he subsequently starts reconsidering his own actions and the government's control of members of this society. Montag goes searching for his own answers. Eventually, he defies society's norms: he hides and reads books, questions the way that he lives, kills Beatty to protect Faber and himself, and changes his very existence because he has changed the way he thinks. By the novel's end, he has joined others who wish to rebuild society and preserve (remember) the contents of books in their minds if the books themselves have been destroyed.
Initially, Clarisse nudges Montag off of the path he is mindlessly following at the story's beginning. By the end, he has become a rebel Clarisse would have applauded, though she might not have recognized him.
But now there was a long morning's walk until noon, and if the men were silent it was because there was everything to think about and much to remember.
Clarisse impacts the end of the book because it is Clarisse who challenges Montag to think, and this transforms his entire life.