Clarisse is the original spark (if you'll permit the fire-related pun) that wakes Montag up. He'd been sleepwalking through his life before she shows him there is more than the programmed routine.
The unidentified old woman in the house of books shows him how much there is to books (that someone would die for them).
Beatty's role is more complex. Beatty first lets him know that he's been watched, and second that he's not alone and that all firemen have been tempted. After that, he essentially tries to counter-brainwash Montag. Finally, when Montag torches him, it is an education for Montag in how far things have gone.
Faber lets him know some of the underpinnings behind the society: how it came to be, what sustains it, and what the risks are.