Jimmy Fallon has spent the last month not commenting on Hollywood's worst-kept secret. After NBC's official announcement on May 12 that he'll slide into the Late Night spot when Conan O'Brien takes over for Jay Leno next year, now comes the even harder part: assuming a role that will be more scrutinized than anything the comedian ever did on Saturday Night Live or in, um, that movie called Taxi. ''My biggest competition is sleep,'' he says. ''If I can get people to choose me to go to sleep to, that's a compliment.'' He hasn't worked out his vision for the new show beyond the desk-and-chairs bit, anyway but he's prepping by hitting the stand-up circuit and analyzing every host from Larry King (whom he impersonated on SNL) to Howard Stern. ''And every time I talk to someone, I sit down to the left of them so I get used to turning that way,'' he cracks. Any chance we'll see a reprise of any of his SNL personas? Probably not, says Fallon, though a certain talk-show skit did unexpectedly groom him for his new gig: ''Barry Gibb has a terrible temper, and I know I don't want to do that,'' he quips. ''Also, if you can do a great falsetto, you automatically get on the show.'' Hey, Prince, you're in!