It happened "after a period of signing 500 signatures," the New Moon heartthrob tells Ellen DeGeneres on her show that airs Friday.
"You kind of get 10 seconds with each person and you never really say anything and I kind of got bored of saying, 'Hey, how are you doing?'" Pattinson tells the talk-show host. "[A fan] said in her 10 seconds, 'What can I do to get your attention?' I was like um, just take your clothes off."
The fan's reaction? "She stood there and frantically started taking her clothes off and got dragged out of the room by security," says Pattinson. "I never felt more terrible."
As for his own body-baring role in New Moon, Pattinson, 23, says, "This is the first time I've ever been proud my physical appearance in my entire life."
But the star, who hit the gym often to get in shape to play vampire Edward Cullen, is shy about going in the buff for another role. Asked if he'd get naked onscreen, he says, "Ah ... maybe, I don't know. Maybe. It's definitely a risk."