Kenan Thompson was "petrified" about meeting Bill Cosby, he says. When dealing with the comic great—and creator of the '70s and '80s cartoon hero Thompson brings to life in the movie Fat Albert—"you want to make him happy." So when he found himself face-to-face with his idol on the L.A. set of Albert last summer, Thompson, 26, went into a routine that has been getting him laughs since he first worked the playground crowd as a kid in Atlanta: his Bill Cosby imitation. Alas, focused on filming, Cosby, who has a cameo role playing himself in the film, didn't even crack a smile. "It was not," recalls Cosby with a laugh, "the time to fool around."

Hey, hey, hey, you can't blame a guy for trying. And for the record, Cosby, 67, liked Thompson's act just fine. "It was wonderful," he says. When Thompson auditioned for the Nickelodeon comedy All That at age 15, producer-director Brian Robbins had the same reaction. "His timing and ability to mimic were amazing," says Robbins. Thompson, who also starred in Nick's Kenan & Kel before becoming a regular on Saturday Night Live in 2003, impressed Cosby with another trait: "He is very well-mannered." Credit his mom, Elizabeth Ann, 55, a nurse in Atlanta. (Dad Fletcher runs a local oil-change business.) "She raised me to be a southern gentleman," says her son. "To this day she keeps me in line."

One day the single actor hopes to keep his own kids in line. But for now, he'll settle for smaller goals: moving out of the spare bedroom in his cousin's Harlem apartment and into his own place—and making people laugh. "I'm a happy person, and I want everybody else to be happy," he says. "Nothing wrong with that."