What's perennial talk-show host Dick Cavett doing in Apollo 13? He'll be darned if he knows. ''I'm happily enjoying a movie, and suddenly I'm in it,'' says Cavett. It's not so surprising considering Apollo 13 star Tom Hanks is a longtime fan. In fact, he crossed paths with Cavett (and John Lennon) last summer in Forrest Gump, too. ''Tom is an idiot savant on the subject of my old shows,'' says Cavett. ''He didn't think he ever missed one he could do guest lists. He must have wasted his entire youth.'' Cavett says he's glad that someone remembers his years on TV, because his 1970 Apollo monologue doesn't ring a bell. ''But,'' he jokes, ''that would go for hundreds of shows.''