Montag goes from being a good citizen of their society wholoved his job. He states that "it was a pleasure to burn"; he wanted to "shove a marshmallow on a stick" in front of the burning house, and went to bed with a "fiery smile still gripped by his face muscles." He took the exact pleasure in his job that his society hoped he would.
He starts to change when he meets Clarisse. For the first time in his life, questions whether he is happy. His safe world had "melted down and sprung up in a new and colorless form." From here on out, he starts questioning things. He starts asking questions about the people whose houses he burns. And, the next house he burns, instead of wanting to callously toast marshmallows, he is highly disturbed. He goes home and can't sleep, then stays home "sick"; he suffers a crisis of sorts. He confronts the Barbie-esque Mildred and her friends. He seeks out Faber, and they make plans for rebellion. He starts openly reading books, defiantly insisting on taking them in.
At the end of the novel, his entire world has changed; "he would not be Montag anymore...and one day he would look back upon the fool and know the fool. Even now he could feel the start of the long journey, the leave-taking, the going away from the self he had been." He finds the courage to run away, and with others, try to rebuild things in a better way.
Montag grows from an unquestioning fireman to a fighter for social change. In the beginning of the novel, Montag is a simple firefighter who does his job of burning books with little question about why the books are such a threat they must be destroyed. The turning point comes when he sees an old woman die with her books rather than live without them. He then wants to know what is so valuable about books that a person to die for them. He then begins to read the books and decides they have great value. Eventually, he escapes from his society and becomes one of the people who memorize books in order to preserve them for posterity. He doesn't have long to wait because his society is destroyed in a war and the people that are left are those that have preserved books, and thus freedom of thought, for all the survivors.