I think the meeting with Clarisse is the beginning of the rising action. Clarisse asks Montag if he is happy. This begins his process of self-discovery. I would say the exposition comes earlier when Bradbury introduces us to Montag and the fact that he has been burning books for over 10 years. We are then introduced to Mildred and her ultimate desire for a fourth television wall. This initial description introduces us to the society and the idea that everyone simply wants to be comfortable. Then, as Montag is going home, he meets Clarisse which signals the beginning of his intellectual journey.