The two observations that Clarisse makes about Montag's conversational mannerisms are as follows:
"You laugh when I haven't been funny and you answer right off. You never stop to think what I've asked you."
Clarisse's two comments reveal truths about both herself and Montag. They show, first, that Clarisse really listens to and notices how Montag responds. He has her full attention, which makes her interaction with him much different from the distracted conversations he and Mildred share.
Clarisse's observations also suggest that Montag is ill-at-ease because he laughs inappropriately at comments that aren't funny. The particular comment he laughs at is her idea, which he immediately dismisses as false, that firemen once put out fires. His response to this idea also shows that he is answering her mechanically, because, as she says, he doesn't stop to think about what she is asking. Both observations indicate that Montag is not used to real conversation that challenges him to think. He has been living in a rote, empty way, a subject he will ponder when Clarisse has left.