That “Fat Ham” achieves its happy, even joyful, ending honestly, without denying the weight of forces that make “Hamlet” feel just as honest, is a sign of how capacious and original the writing is, growing the skin of its own necessity instead of merely burrowing into Shakespeare’s. It’s also a sign of how beautifully the cast brings the writing to life. Everyone is excellent, and Thomas’s loud-lady-in-the-pew-behind-you routine is flat-out hilarious. But Spears, with his minute calibrations of feyness and fierceness, holds the whole thing together. In his scenes with Crawford, especially one in which Tedra pleads with Juicy to hold it together — “you don’t get to go crazy” — he lets us see how a character creates and re-creates himself in real time.