Some actors prepare for a role by using Method. Then there's Jim Carrey, who goes to the dentist. As Lloyd Christmas, a.k.a. Dumb of Dumb and Dumber, the rubber-faced comedian sports a chipped front tooth, which, he says, is real. ''Clark La Prairie jumped on my head in grade school detention,'' says Carrey, explaining the source of the old injury. ''The nuns sent me home with my tooth in an envelope.'' So when Carrey decided that his irregular chopper would complement his character's goofy grin, he had his cap removed. ''De Niro gains weight. I file my teeth off,'' he says. It was worth the effort, though: Dumb will likely be Carrey's third consecutive hit, following Ace Ventura: Pet Detective and The Mask. Does his dentist have points?