In Fahrenheit 451, the first event that sparks Montag's transition is his encounter with Clarisse. At the conclusion of their conversation, Clarisse asks Montag if he's happy. This happens in the first 10 pages of the book. After their conversation, Montag obsesses over Clarisse and her questions.
He felt his smile slide away, melt, fold over and down on itself like a tallow skin, like the stuff of a fantastic candle burning too long and now collapsing and now blown out. Darkness. He was not happy. He was not happy. He said the words to himself. He recognized this as the true state of affairs.
A second significant moment in Montag's transformation occurs during his visit with Faber in Part Two. The very fact that he goes to Faber seeking a teacher indicates his increasing desire to learn more about books and ideas of free thought and rebellion. Montag leaves with the radio receptor in his ear. And for the first time since his transformation, he doesn't feel alone in his rebellion. Faber reads to him as he walks home.
Far away across town in the night, the faintest whisper of a turned page. "The Book of Job." The moon rose in the sky as Montag walked, his lips moving just a trifle. (83)