What does Montag think about Beatty's visit in Fahrenheit 451?
This part of the story is sort of the conclusive moment of Montag's resolve to change himself and possibly the world around him.
Through his questioning of things (namely, the history of firemen) it is clear that Montag is not satisfied in his job as everyone else seems to be. He is also unsatisfied by life.
After Beatty's visit - it is as if Montag's questions are more fully answered. He was dabbling in his search for answers (with Clarrisse, with the books he's stolen, with his scratching at the surface for answers) and by the time Beatty leaves Montag has resolved not to go back to being a fireman. This decision shows that Montag has gained confidence in himself and his ideas that something is wrong with society. He further proves this confidence when he shows Mildred the books he's stolen, and then convinces her to help him search through them to find answers.
He keeps a straight face and plays along with Beatty - but it is clear that Montag does not believe Beatty's explanation - nor will he be convinced by it.