Matthew Guerrieri is not ready for his close-up. A freelance classical music critic who writes regularly for The Boston Globe, Guerrieri is sitting down Thursday with Stephen Colbert, and he’s very nervous. “I just keep reminding myself it’ll be fun,’’ he said. Guerrieri, who lives in Framingham, is going on “The Colbert Report’’ to talk about his book, “The First Four Notes: Beethoven’s Fifth and the Human Imagination,’’ which is getting a lot of love from critics. As anyone who watches the Comedy Central show knows, Colbert knows a thing or two about music, and that appeals to Guerrieri. “You can tell he’s interested in music and musicians and there’s real conversation,’’ he said. “But it’s TV. I’m so used to hanging in the back the auditorium, taking notes, and then going home and writing. I’m just gonna go there and have a good time.’’ . . . Pulitzer Prize-winning author (and MIT prof) Junot Díaz will be on “The Colbert Report’’ March 25.