The previous poster has it right and one of the important things that happens is when Montag meets Clarisse as her incessant questions and her completely different manner than the rest of the people around him drives him to also begin to question things in his own life. As he allows the questions to enter his mind, he sees the absurdity of his wife's existence and starts to lose the satisfaction he felt earlier in doing a good job of burning, etc.
The burning alive of the woman and his eventual perusal of a number of books are what push him over the edge and he begins to fight actively against the firemen and what they stand for eventually abandoning the society completely to try and start over.
The central character of the novel and its hero. Fahrenheit 451 is about the transformation of Montag from an obedient servant of the state to a questioning human being. Montag begins to question his society and his place in it. He also starts to think about his wife and how she spends all her time gossiping. Montag begins to read the forbidden books and realizes that the societal rules are not what he believes in.