Insect Aside: Michael Shannon’s character breaks for more aphid hunting.

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Bug originated as a piece of theater—and arguably still is one. Tracy Letts’ adaptation of his own play, directed by The Exorcist’s William Friedkin, will bore the hell out of anyone approaching it as a movie. Especially one of the horror variety: “Experimental” is the description that sticks to Bug best, and though it’s got shades of other far-out fare like David Cronenberg’s work, the material is so deliberately paced and deeply psychological that it requires little more than a black-box theater, not a film set.

A puffy-eyed Ashley Judd stars as Agnes, an Oklahoma barmaid living in a run-down motel. Agnes’ social life consists of drinking alone and getting crank calls from her estranged husband, Jerry (Harry Connick Jr.), who recently got out of jail. Jerry eventually shows his face to make threats and push her around, but she doesn’t worry too much, because the night before a friend introduced her to Peter (Michael Shannon). Peter’s weird stare matches his weird conversation—“They want you to know they’re there,” he says of “machines” that whir in the night—but because he has nowhere to go, Agnes allows him to crash on the couch. Love comes to town. So do bugs.

Allegedly, at least. Letts’ story is ultimately an extreme cautionary tale about how a little bit of passion can make you do magnificently fucked-up things. Agnes at first can’t see the critters that Peter claims are biting him in bed. (He says they’re aphids, after patiently explaining the differences between fleas, lice, ticks, and the like.) But soon she feels them too and believes Peter when he confides that the military actually planted the bugs in his blood. Now it becomes the two of them against the world: Agnes accuses her friend R.C. (Lynn Collins) of turning on her when R.C. insists that Peter is bad news, and she believes that the motel manager is part of a conspiracy when he claims no other rooms have reported insect problems. The couple barricade themselves in their room and develop a logic all their own.

Even if you approach the film with a made-for-stage mindset, Bug has its problems. Letts seems to have devoted most of his efforts toward developing Peter, and his character is terrific—his initial social awkwardness borders on the autistic, and the tangles of theories he slowly lets Agnes know are clouding his head are paranoid-schizophrenic intricate. You like him, though, because the complexity of his thoughts first makes him seem smart. And the way he patiently explains them to his backwoods girlfriend without talking down? Compared to the brutish Jerry, Agnes has found a prince. But the attention to Peter makes the rest of the film suffer: Mainly, Agnes catches crazy too fast. Though Agnes’ character has a psychological crack waiting to bust open—she and Jerry had a son who disappeared when she took him grocery shopping 10 years ago—her spiral into psycho-shrillness is unbelievably instantaneous. Letts doesn’t help her anti-heroine any by stuffing words into her mouth, with one monologue in particular running an excruciating length.

Judd’s performance is manic and raw; you may dismiss Agnes as silly, but the sweet-faced actress is uncharacteristically intense. Shannon, however, is the film’s redeemer as he reprises the role he originated on stage. His wide-set eyes first offset his strong, squared jaw and suggest his Peter is merely a gentle giant. Later, though, the madness in Shannon’s expression combines with the drying blood all over Peter’s wound-covered body to give the impression of a monster. It’s this character, not the presence of creepy-crawlies, that provides the true horror in Bug. The problem is that you may have to mentally strip away the cinema to see it.