This film from William Friedkin (“The Exorcist”) is being marketed as a conventional horror movie, which is absurd. Though it certainly has its share of horrific moments—more so, in fact, than run-of-the-mill Hollywood shockers—Tracy Letts’s adaptation of his 2004 off-Broadway play is basically a brutal portrayal of paranoia. The film to which it might most properly be compared is David Cronenberg’s “Spider.” But while that picture was far subtler, portraying the deranged mind from within, as it were, and doing so with an almost unparalleled form of cinematic severity, this one is a view of paranoia from the outside, told in a furious and florid style. And though not the equal of Cronenberg’s film, it’s an intriguing and powerful alternative to it.

There’s more than a hint of Sam Shepard in the set-up, milieu and undercurrents in the movie. Agnes White (Ashley Judd) is a waitress living in a dismal motel room in the Oklahoma boonies, and bothered by repeated telephone calls in which no one speaks when she answers. Eventually it becomes clear that she’s afraid they’re coming from Jerry Goss (Harry Connick, Jr.), her ex-husband just out of jail and a threatening sort of fellow. It’s also eventually revealed that their troubled history together includes the loss of their young son, who unaccountably disappeared during a shopping trip with his mother. And Jerry himself eventually turns up at the motel.

Before he does, however, others have entered the scene. One is R.C. (Lynn Collins), Agnes’s lesbian friend, and along with her Peter Evans (Michael Shannon), a strange, oft-spoken young man she’s met at the bar. Agnes and Peter quickly develop a quiet rapport, and she offers to let him stay the night. Gradually a halting romance between them emerges, though his curiously controlled mien and the occasional hint he lets drop about his fears suggest that all is not as placid within him as it might be.

As the piece moves into what’s clearly its second act, the susceptible Agnes has been fully drawn into Peter’s extreme paranoia, expressed in his belief that as a soldier in Iraq he was the subject of experimentation that involved the injection of some tiny bugs under his skin (presumably to control and/or track him), and the pair lock themselves inside the room against the dangerous world outside. Their already paralyzing fear is only increased when a government doctor (Brian F. O’Byrne) shows up seeking Peter. The condition in which he finds them, and their reaction to his intrusion, are not pretty.

To appreciate “Bug” you have to be willing to immerse yourself in somebody else’s fever dream, and especially in its final stages, it’s not a pleasant experience, although it is drenched in—among other things, some of them very red—a streak of mordant humor. Though not pleasant, however, it’s certainly compelling. The only issue is how many viewers might care to be compelled in this way.

It’s also marked by two extraordinarily strong performances. Judd, after years of starring in lousy action-heroine movies, recently reminded us, with her subtly understated turn in “Come Early Morning,” that she’s a fine actress, and here she’s ferocious and courageous, refusing to hold anything back. And Shannon, who played Peter on stage in the original production, is a revelation, anchoring the film with a turn that captures the oddity and obsessive drive of the character without ever stumbling into mere caricature. The claustrophobically close-in camerawork of Michael Grady doesn’t flatter them in the Hollywood sense, but it captures the frantic movement that accentuates Friedkin’s vision as well as the intensity of lead characters’ view of the world. The supporting cast delivers soundly, with Connick working up a fine sense of menace and O’Byrne one of almost nonchalant bemusement in the face of the outrageousness he sees around him.

From the simple narrative perspective, there are questions implicit in “Bug.” For example, was that lost child that destroyed Agnes and Jerry’s life real, or imaginary (as in “Who’s Afraid of Virginia Woolf”)? But the big one, of course, is whether Peter and Agnes really have little critters crawling around inside them. Probably not—but though the characters in it might not actually have bugs under their skin, Friedkin’s creepy film will certainly get under yours, for good or ill.