Several things spark Montag's transition in Fahrenheit 451, by Ray Bradbury, that eventually change him from follower to hero. Clarisse McClellan's ideas first make Montag start to think.
Montag is a fireman who dutifully burns down houses of people who possess books—which is against the law in the futuristic novel. Initially, he is quite happy with his job:
It was a pleasure to burn.
It was a special pleasure to see things eaten, to see things blackened and changed.
Ironically, Montag likes to see the changes burning brings, but nothing in his life changes and he is perfectly satisfied with it until Clarisse begins to ask him questions. They are not difficult queries. For example, she asks him:
Do you ever read any of the books you burn?
He laughs and responds without thinking:
That's against the law!
Clarisse points things out that he hasn't thought about in years: the morning grass.
"There's dew on the grass in the morning."
He suddenly couldn't remember if had known this or not...
Because Montag never questions anything that society does or expects of him, he is quite surprised by Clarisse and not only her questions, but also her forthright manner of speaking. On another occasion, Clarisse describes how everyone drives—so fast that things along the road are nothing but blurs of color—the flowers, cows, etc. Only the billboards showing government-approved messages are visible. The government doesn't want people noticing things because it doesn't want people asking questions. And that is why books are banned and entertainment controlled—to prevent original thought.
When Clarisse asks Montag if he is happy, his first response is an unthinking "yes." However, he realizes upon returning to his home—built to manipulate and dull the senses, that he isn't happy in the least. If Montag is deadened to the world, his wife is worse: Mildred is so numb, that she overdoses on pills one evening and is totally unaware the next day that Montag had to have her stomach pumped to save her life. These things cause Montag to think and to doubt.
Perhaps the most devastating and life-altering experience Montag faces is the house at 11 North Elm that they are called to burn.
Always before it had been like snuffing a candle. The police went first...so when you arrived you found an empty house...
This time, the house is not empty.
...tonight, someone had slipped.
Before, Montag recalled...
You weren't hurting anyone, you were hurting things!
In this case, the owner is there as the men throw out her books and douse them with kerosene to burn. She tells them they cannot have her books, and she touches them with great reverence. She refuses to leave the house. When Montag's boss, Beatty, begins to count down, she tells him not to bother, and she opens her hand—which holds "an ordinary kitchen match." Telling Montag to go, she ignites the match, destroying not only her books, but also herself. She refused to live without her books.
This changes Montag irrevocably. He follows no more, but begins to hoard books himself. At the end, he is on the firetruck thinking:
How can I go at this new assignment, how can I go on burning things? I can't go in this place.
When he realizes they have come to burn his house, Montag responds to Beatty's taunts by killing him—because he cannot live as he has in the past. He escapes and joins others like him, defying society and becoming a heroic figure.