Theater criticism should be visceral, at least on some level, an articulation of that fierceness and passion. I usually do a fair amount of research before I see a show - on the history of previous productions (if it's a revival) and the creative team.
It's nowhere near as intense as what I imagine an actor experiences backstage, but I feel a fluttering nervousness before a curtain goes up on a play. I mean, any play, anywhere - on Broadway or the Bowery or in a church basement.