Audiences know him as a fiery-haired, hyper, late-night goofball, full of energy and banter and wit. But Conan O'Brien's staff knows the other side of the comedian, the dark side. It's the side that's earned him the behind-the-scenes nickname "Mean Conan."

O'Brien, 47, says his colleagues think he's even funnier when the devil within is unleashed. But he's also nastier. When he slumps into one of his black moods, he becomes harsh, biting and cruelly sarcastic, not the amiable, chatty guy we see on his TBS show 'Conan.'

"I'm really hard on myself," he told The New York Times this week when asked to describe the sinister part of his personality. "I tease people constantly. I physically fight my writers, and they fight me back."