Early in the book, on the second and third pages, Montag has a premonition of something different in the vicinity of the sidewalk near his house, as if "a wind had sprung up from nowhere, as if someone had called his name." Shortly thereafter, he meets Clarisse. Montag's encounter with Clarisse is the beginning of his transformation. Clarisse is a free-thinker and Montag can hardly keep up with her. He laughs when she suggests that firemen used to put fires out. Clarisse doesn't watch the 'parlor walls' and she seems only to engage in thought-provoking observations and conversation. At the end of their first conversation, she asks Montag if he is happy. After she leaves, he is consumed by her question and subtly realizes that she got him to think about himself, to question himself. He remarks that people rarely do this because they are so self-absorbed, passive, and drawn into themselves:
How rarely did other people's faces take of you and throw back to you your own expression, your own innermost trembling thought?
After Clarisse disappears in Part 1, Montag's self-awareness increases. While playing poker with the other firemen, he notices they all look the same, as if they are all mindless clones of one another. Entertaining his new found skepticism, then he asks Beatty if firemen used to put fires out. The other firemen scoff at this.
At the next fire, Montag witnesses a woman who refuses to leave her books and chooses to stay and burn with them. Upon telling Mildred about this, Mildred is dismissive and can only talk about the parlor shows. Montag slowly begins to realize that she wants no part of his motivation to question they way they live. During that previous fire, Montag steals a book; this is also a significant moment of his transformation: defying the law in seeking knowledge in books. Montag continues to take books from the fires, essentially building his arsenal of knowledge which he will use to enlighten himself.
In Part 2, Montag seeks to continue his education so he contacts Faber. By this time, it is clear that Montag is questioning everything about the society he lives in. He goes to Faber to serve as his mentor.
Montag's dramatic changes occur mostly in Part 1. More than anything else, Montag's changes are inspired and initiated by his conversations with Clarisse. Montag is increasingly disheartened with how isolated Mildred is and he is particularly struck when she thoughtlessly forgets to tell him that she'd heard that Clarisse was killed (page 44 in the Ballantine edition).