Fahrenheit four-five-one is the temperature at which book paper catches fire and starts to burn.
Book paper actually combusts at 450 degrees celsius, but Ray Bradbury (the author of the book this movie was based upon) thought fahrenheit sounded more powerful. Book paper catches fire at 842 degrees fahrenheit.
[to Linda] You spend your whole life in front of that family wall. These books are my family. When did we first meet? And where?
The Captain: What does Montag do with his day off duty?
Guy Montag: Not very much, sir. Mow the lawn.
The Captain: And what if the law forbids that?
Guy Montag: Just watch it grow, sir.
Guy Montag: Hasn't this uncle of yours ever warned you never to speak to strangers?
Clarisse: No. He did say once if anyone asked how old I was to say I was 20 and light in the head. They always go together.
Guy Montag: Light in the head?
Clarisse: Mm-hmm. Loopy. Crazy. Anyway, you don't frighten me.
Montag: Why should I?
Clarisse: No reason really. The uniform, I suppose.
Clarisse: Is it true that a long time ago, firemen used to put out fires and not burn books?
Guy Montag: Really, your uncle is right, you are light in the head. Put fires out? Who told you that?
Clarisse: Oh, I don't know. Someone. But is it true did it?
Montag: Oh, what a strange idea. Houses have always been fireproof.
Clarisse: Ours isn't
Guy Montag: Well, then, it should be condemned one of these days. It has to be destroyed, and you will have to move to a house that is fireproof.
Clarisse: You don't like the books then?
Guy Montag: Do you like the rain?
Clarisse: Yes, I adore it.
Linda Montag: Did you see that? Cousin Claudette's got a bouffant tonight.
Guy Montag: Who?
Linda Montag: Cousin Claudette.
Guy Montag: Who is Cousin Claudette?
Linda Montag: The cousin announcer, the one you don't like.
Guy Montag: I don't like any of them.
Guy Montag: Well then it wasn't the analyst. It was the staff that wanted to get rid of you because you are different. Look at that fellow over there.
Clarisse: What's he doing?
Guy Montag: That's the information box. He can't make up his mind.
Clarisse: What's he want to find out?
Guy Montag: He doesn't want to find out anything. He knows someone who has books. So he got hold of the person's picture and number, and is going to drop it into that box.
Clarisse: But he's an informer!
Guy Montag: No, he's an informant. Look at him. Like someone circling around a woman.
Clarisse: He's putting something in his mouth.
Guy Montag: It's a stimulant to work up his nerve.
Guy Montag: What?
Clarisse: How did it come about? What made it begin? What made you want to do – How could someone like you be doing this kind of work? I know everyone says that, but you! You're not like them. When I say something to you, you look at me. Why did you choose this job? For you it doesn't seem to make any sense.
Guy Montag: Do you remember what you asked me the other day? If I ever read the books I burn? Remember?