In Fahrenheit 451 by Ray Bradbury, Montag meets Clarisse on the third page. She is unlike anyone he has ever known. While he and his wife and "friends" are automotons, doing everything as directed by the government, Clarisse and her family actually do what the men down the river do at the end of the novel: they try to remain connected to the past, remembering things that matter, things that uplift the human experience. They are not a part of the unnatural flow of the community, but keep to themselves and find comfort in each other.
Clarisse encourages Montag look at the world through her eyes—she asks him if he thinks about things, and if he is happy. Montag finds her very odd. She explains that she does not go to school: no one misses her, saying that she's not social. She disagrees and then the reader realizes that it's really because she won't "behave." She says:
But I don't think it's social to get a bunch of people together and then not let them talk, do you?
Clarisse is curious about the world. She watches people to see how they act. She listens to people everywhere, but says that they don't talk about anything. She notices blades of grass and flowers, which no one sees because no one drives slow enough to do so. They have become desensitized. In sharing all of these things, she gets Montag to think about "thinking." She pushes him to notice the world around him and to ask questions.
The purpose of Clarisse and Montag's meeting is to get him to engage more with the world around him, so he begins to notice things and ask his own questions. He becomes concerned with the apathetic way his wife lives. He questions the validity of what firemen do.
Once the first idea is planted, Montag cannot help himself—he secretly collects more and more books. He questions society's laws. Soon, he is ready to make a break from this oppressive society of which he is a part.
However, when he faces Beatty at the end, his boss brings up Clarisse to Montag, sneering at the way Montag fell under her influence.
"You weren't fooled by that little idiot's outine, were you? Flowers, butterlfies, leaes, sunsets, oh, hell! It's all in her file...A few hrass blades and the quarters of the moon. What trash. What good did she ever do with all that?"
Montag sat on the cold fender of the Dragon... "She saw everything. She didn't do anything to anyone. She just let them alone."
"Alone, hell! She chewed around you, didn't she?..."
While it would seem at first that Clarisse gave Montag a desire to ask questions and engage with the world around him rather than passivley passing the time, as Mildred did, Beatty's knowledge of Clarisse and her time spent with Montag would indicate that Montag was being watched for a long time. Beatty even admits that he knew that Montag was involved with books before they came to burn his home.
However, the fact that Beatty tells Montag about Clarisse, along with the sense the reader gets with Montag's lament, "She didn't do anything to anyone," makes the reader wonder. Can we infer that the car that hit Clarisse may not have done so accidentally? After Montag kills Beatty and runs away, he has a moment of clarity, realizing that Beatty wanted Montag to kill him. Perhaps he mentioned Clarisse as just another way to drive Montag to end Beatty's life. So that Clarisse and Montag's meeting may well have served two purposes.