When Montag first meets Clarisse, he is in the middle of a feeling of unease about his life, although he cannot put it into words and it has not started to affect his work. Since Clarisse is already an individual, she finds his conversational style strange:
She glanced quickly over. "Why are you laughing?"
"I don't know." He started to laugh again and stopped. "Why?"
"You laugh when I haven't been funny and you answer right off. You never stop to think what I've asked you."
(Bradbury, Fahrenheit 451, Google Books)
Montag laughs regardless of whether her words are funny, because that is how he is conditioned; since television programs are not about ideas or story, but about pure, meaningless emotion, he applies emotion incorrectly and indiscriminately. He also answers without thinking about his response; again, the culture has conditioned him to simply parrot the words of other people, giving the "correct" response without any worry as to its actual meaning. The fact that Clarisse notices these conversational oddities kick-starts his brain, allowing him to think about his words for the first time.