Michael C. Hall eats a dinner of of kale salad and roast chicken in his purple dressing room at the Belasco Theater on 44th St. in New York City. It's a December evening, and his protective little rescue dog, Sal (short for Salamander), looks at him expectantly, waiting for a crumb.

"If I don't force myself to eat at certain times of the day, I'll disappear," Hall says apologetically between bites.

Hall,...