Clarisse questions society's conventions and rules, and gets Montag questioning too.
Clarisse thinks about whether or not people are happy, but no one else in their society does. She introduces herself as seventeen and "crazy," saying that the two go together, according to her uncle. What makes Clarisse different, though, is that she stops and thinks about things. She comes by it honestly. Her family is different. Other people in Clarisse's society do not stop and think about things the way they do.
When Montag meets Clarisse for the first time, he can immediately tell she is different because she seems to notice things that others do not notice. She stops and slows down in a society where going fast is expected.
Isn't this a nice time of night to walk? I like to smell things and look at things, and sometimes stay up all night, walking, and watch the sun rise."
They walked on again in silence and finally she said, thoughtfully, "You know, I'm not afraid of you at all." (Part I)
Montag is surprised that Clarisse might be afraid of him, but she tells him that many people are afraid of firemen. It makes sense, since they barge into people's homes and burn them down. Montag tells us later that the people are usually not there, because they have been arrested before the firemen arrive at the houses. He has therefore not had many interactions with the public, and really hasn't considered public perception of his job before.
While Montag hasn't thought about what being a fireman means, Clarisse clearly has. She asks Montag if he has read the books that he burns.
"Do you ever read any of the books you bum?"
He laughed. "That's against the law!"
"Oh. Of course."
"It's fine work. ...
They walked still further and the girl said, "Is it true that long ago firemen put fires out instead of going to start them?"
"No. Houses have always been fireproof, take my word for it."
"Strange. I heard once that a long time ago houses used to burn by accident and they needed firemen to stop the flames." (Part I)
Clarisse gets Montag to think. She asks him if he is happy, and he has never thought about happiness. Why does she think about it, but he does not? Montag is a product of his society. He falls into lock-step, and does what he is told. Clarisse, on the other hand, has an unusual family and they have unusual ideas. She is young, and does not like what she sees. She complains about her generation, driving fast and killing for fun. To Clarisse, society does not produce happiness. When she runs into a real firemen, she asks him if he is happy because she wonders if burning books is wrong.
Sadly, Clarisse meets an unhappy end. Montag barely even finds out about it, since her life is a footnote to those around her. In his society, death is nothing. She gets run over by a car, a "beetle," and this is so common that it is barely worth mentioning. The irony is that because of the conversation with Clarisse, Montag is now one of the only other people that would have cared. Clarisse would have cared, but now she is dead.