The sudden death of Clarisse in Part One has a huge impact on Montag. It is Mildred who breaks the news to Montag and, the morning after, Montag develops "chills and fever" which stop him from going to work. He also sleeps "five hours" later than usual. This is significant because it is the first time Montag has ever been sick, which Mildred points out to him.
Clarisse's death also provokes an identity crisis in Montag. He starts talking about quitting his job, and suddenly becomes conscious of the moral implications of his job as a fireman, saying,
Last night I thought about all the kerosene I've used in the past ten years. And I thought about books. And for the first time I realized that a man was behind each one of the books.
His absence from work leads to a visit from his boss, Captain Beatty, who realizes Montag is beginning to question his role in society. Clarisse's death set Montag on the path to rebellion.