Clarisse's role in changing Montag's view about books--and life in general--is monumental and all important. Without Clarisse, Montag might have lived "happily" as a fireman the rest of his days, with only an inkling of curiosity about books that acted itself out in the occasional snatching and hiding of the rare book. When Montag meets Clarisse, he is walking back from work, self-satisfied with his day of burning. After his first visit with her, when she asks the all-important question, "Are you happy?" everything changes. He realizes that he isn't happy at all, and that she was; he didn't know how she got to be that way, but he suspects that books had something to do with it. Clarisse is the first to ask him,
"Is it true that long ago firemen put fires out instead of going to start them?"
The thought had never occurred to him, and starts him thinking. She also asks him if he ever reads any of the books they burn. This question too gets him thinking. Her inquisitive mind, and intriguing questions start him on a path of exploration and answer-seeking that ends with reading books.
Without Clarisse's calm, luminescent happiness and thought-provoking questions, Montag might not have been guided towards book as he was. The first step was realizing that he was unhappy; the next was that SHE was; the next was that his job might not have always been what it was, and the last was that maybe the answer to his misery could be found in books. Clarisse is responsible for setting him on that path. I hope that helped; good luck!