At the beginning of the novel, Montag is fully satisfied with his life as a fireman. He takes pleasure in burning the books and has a visceral high from his job, somewhat like an addiction:
He knew that when he returned to the firehouse, he might wink at himself, a minstrel man, burntcorked, in the mirror. Later, going to sleep, he would feel the fiery smile still gripped by his face muscles, in the dark. It never went away, that. smile, it never ever went away, as long as he remembered.
In this role, Montag is a tool of the authorities. He has been conditioned and brainwashed to believe that burning books is a righteous vocation. He has also been taught that creative thinking and questioning things are practices that will only lead to dysfunction and unhappiness. So, he doesn't think of such things.
The first instance that sparks the beginning of change in Montag occurs with his first conversation with Clarisse. After bombarding him with questions, she leaves asking him if he is happy. He is struck by the fact that she got him to think about himself in a profound way:
People were more often-he searched for a simile, found one in his work-torches, blazing away until they whiffed out. How rarely did other people's faces take of you and throw back to you your own expression, your own innermost trembling thought?
He goes home and has to deal with Mildred's overdose. Then he actually says, "I don't know anything anymore." He is clearly uncertain about things at this point, whereas at the beginning of the novel, he is comfortable and happy in his ignorance.
Another conversation with Clarisse moves Montag to have more questions. He sees a woman choose to burn with her books during a raid on her house. This affects him deeply. In a later conversation with Millie regarding this event, he says, "There must be something in books, things we can't imagine, to make a woman stay in a burning house; there must be something there. You don't stay for nothing." Montag steals a book from the raid.
His curiosity about books and knowledge increases. He takes more books. He seeks out advice from Faber, a college professor. Clarisse's disappearance also is a clue. He begins questioning a society that would eliminate a girl simply for being curious about life. In a culminating moment, he reads poetry to Millie and her friends. This leads Millie and one of her friends to turn Montag in. But by this point, he has his mind set on becoming the intellectual revolutionary. In the end, he must flee his own home and happens upon the other traveling intellectuals.