3 stars

Cast: Ashley Judd, Michael Shannon, Harry Connick Jr., Lynn Collins.

Director: William Friedkin.

Running time: 102 minutes.

Rated R: Vulgar language, sexual situations, nudity, violence, gore, drug use, adult themes.

“Bug” is being marketed as a horror movie (“From the director of “The Exorcist’!” the ads proclaim) but it isn’t really a horror movie per se, although it has a number of horrific moments, and it gets under your skin the way genuinely disturbing art sometimes does.

Based on Tracy Letts’ 2004 off-Broadway play, “Bug” was directed by William Friedkin, who once did, in fact, make “The Exorcist” (and “The French Connection” and “To Live and Die in L.A.”) but more recently brought us “The Hunted” and “Rules of Engagement.” One of Hollywood’s most powerful auteurs in the 1970s, Friedkin has faded into gun-for-hire obscurity over the last decade, which explains why “Bug” has an uncompromising, anything-goes daring: Friedkin, 71, has nothing to lose at this point, and he has made this low-budget, brazenly over-the-top picture strictly on his own terms.

He has also rounded up a group of actors who help ground the lunacy of this whacked-out story with a grave and dramatic intensity that prevents it from veering into camp. “Bug” centers on Agnes (Ashley Judd), a cocktail waitress who has holed up inside a rundown Oklahoma motel room, hoping to hide from her abusive ex-husband (Harry Connick Jr.), who has just been released from prison and has started harassing her with crank phone calls.

A mistrustful loner by nature, Agnes finds a kindred spirit in Peter (Michael Shannon), a drifter who gradually wins her over with his amiable, nonthreatening personality. It only takes the appearance of a tiny bedbug, however, for that demeanor to reveal deep, troubling fissures.

There aren’t a lot of actual insects crawling around in “Bug.” It’s the movie itself that feels like it’s creeping around in your head. Friedkin, who seems energized by the opportunity to work on a small scale, uses the grimy interior of the motel room as a mirror for his characters’ state of mind, which makes things feel all the more claustrophobic as delusional paranoia and schizophrenia begin to surface.

Judd and Shannon are fearless in portraying Agnes’ and Peter’s growing psychoses, and their performances are so strong that even lines such as “I am the super mother bug!” come off as disturbing instead of ridiculous. There’s nothing about “Bug” you’d call pleasant – this is essentially a study of mental illness – but this dark, intense picture is the best and most vibrant movie Friedkin has directed in decades.