EVEN AS A TODDLER, MACKENZIE Astin "was a witty kid" with "a devilish look to him," recalls his mother, actress Patty Duke, 50. When he'd throw a tantrum, though, "he'd kneel on all fours and bang his head on the floor. It made a horrifying sound, and it was a hideous sight. I would just lift him up and move him to a softer spot."

Two decades later, Astin has left the family carpet and is sitting pretty in Hollywood. Now 23, he has scored parts in two recent movies—playing Chris O'Donnell's pal in In Love and War and Shirley MacLaine's grandson in The Evening Star—and he'll be making his leading-man debut opposite Jennifer Aniston in Dream for an Insomniac, the story of a romantic. "I don't believe in representing the kind of behavior that isn't good for society," says Astin. (Ah, youth.) "For now, playing a nice guy is where I'm at."

Not too long ago, however, he didn't aspire to play anybody. After a three-year run on the 1980s sitcom The Facts of Life, Astin returned to finish high school and planned to go to Johns Hopkins, the alma mater of his father, actor John Astin, 66, to study journalism. But with an Oscar-winning mom (for The Miracle Worker), a TV-veteran dad (Gomez on The Addams Family) and a working-actor brother (Rudy's Sean Astin, 25), the cameras finally proved a bigger lure. "My dad was a little bummed out when I decided not to go," he says, "but my mother was glad to have me back on the team."

Off the set, Astin shares a West L.A. ranch-style home with two buddies and a pair of cats. Currently girlfriend-free, he reads a lot (favorite authors: Kurt Vonnegut and J.D. Salinger) and stays in touch with his parents, who divorced in 1985. As for that old baby wit, he clearly still has mom's number. "Yesterday he left a stupid message on my machine," says Duke. "I didn't get it, but I laughed anyway."