Possession has been a lifelong preoccupation for William Friedkin. He’s addressed it head-on as both fiction and fact, but Bug sees him take a more oblique route. Here’s the story of a man so thoroughly possessed by paranoia that his delusions are contagious. One demon leaves one body to enter another, but an obsession is Legion.

Every Michael Shannon performance is arguably his best, but this is a film tailor-made for his fascinating idiosyncrasies. Aphid and spastic, his body language stops short of actually turning into a freaking insect.

Ashley Judd, however, has a more challenging role, because not only does she have to sell the transition from sane to crazy, but then she has to catch up with Shannon, go toe-to-toe with him, match his manic intensity — and I’ll be damned if she doesn’t; Judd digs deep and reaches a place of utter darkness and desperation. She stares right into the abyss and doesn’t flinch.

Everybody is in point, though; Friedkin and screenwriter Tracy Letts, pull off the rare double-turn (to use wrestling terminology). Harry Connick Jr., who plays Judd’s character’s abusive ex, is all brawn and no brains, while Shannon starts out helpless and meek (his patented, infallible calm-before-the-storm routine); we begin to dread the seemingly inevitable moment when Connick beats Shannon within an inch of his life, only to end up wishing that the former would slap some sense into the latter.

The only problem with this film is that it builds so much momentum it just can’t help crashing and burning. It’s so climactic that it actually becomes anticlimactic. There’s no resolution, no catharsis. For all its shock and awe, The Exorcist allows itself a hopeful, optimistic coda; Bug lacks such an escape valve. This time, the Devil wins.