It’s difficult to be thrilled by just about anything that happens in William Friedkin’s “Bug,” mostly because pity, rather than paranoia, is the overwhelming emotion — pity for these broken shells of human beings who are cast adrift without a friend to lean on, pity about their inevitable slide toward madness. Everything about this pseudo-thriller is being blatantly mismarketed; posters in just about every subway station across the city are billing it as a horror bonanza “from the director of ‘The Exorcist.'”

The on-screen product, however, could not be further from that promise. “Bug” plays out as a quiet, and plodding psychological drama that never quite allows the audience into any character’s head. In the hands of a different director, using different actors, and incorporating different dialogue that humans might actually say to one another, “Bug” would at least have the potential to succeed as a drama of delusion. One can see how the story could be an effective case study in claustrophobia and cult mentality, a shocking step-by-step procedural that follows two healthy human beings down the road of dependency, paranoia, and violent schizophrenia.

But, alas, that story will have to wait in the wings, because what we have in “Bug” is the amateurish variation on that theme, poorly paced, awkwardly structured, and bizarrely acted. Things start with an aerial shot of rural farmland (a view from a helicopter that will repeat throughout the film), followed by an aerial overview of the motel room where almost all of the story’s action takes place (from the outset, “Bug” feels like a one-set theatrical production, which shouldn’t seem like a surprise since the script is adapted by Tracy Letts from his own play).

Listening to the thwap-thwap-thwap of the chopper is Agnes (Ashley Judd), who has basically cashed in her chips and given up on life. Depressed and disheveled, living in a dank and dirty motel, she chugs vodka and flirts with cocaine as she waits out her days. But before she can do that, her nights start to become dominated by a series of mysterious phone calls that offer nothing but static on the other end of the line. The more it rings, the more Agnes becomes convinced that it’s Jerry (Harry Connick Jr.), who’s probably calling to let her know that he’s returning home from prison. She doesn’t seem too excited.

Considering what does pass for excitement in Agnes’s life, it’s little surprise when she meets Peter (Michael Shannon) at a club and invites the Army man to stay over at her apartment — on the couch. He agrees, and slowly one night turns into the next day, and then another. As the days pass, he seems to match her level of awkwardness and fragility. While she talks about the pain in her past, he admits he doesn’t get along well with people, and that he’s lost interest in romance and sex. Soon Peter has moved in, and when he awakes in bed one night, yelping that he’s been bitten by a bug, Agnes trusts him even though she can’t see the bugs that he claims have set up camp in the mattress.

As the days pass, Peter ups the ante. First he claims there’s one bed bug, then there’s a swarm, and eventually a whole colony, living inside him — a weapon used by the government against this AWOL soldier. Their fear slowly turns to paranoia, then makes the leap into violence; even a pizza deliveryman represents a threat.

But a horror film this movie is most certainly not. It attempts to be a psychological drama, but is repeatedly tripped up by dialogue that is either too ambiguous or too literal, and by two dismal performances that leave Peter and Agnes seeming by turns stiff and exaggerated.

Mr. Shannon, in a monotone role, seems stuck in one gear — awkward — and while that makes Peter’s obvious mental disorder seem more plausible, it makes his wooing of Agnes, not to mention his convincing her about an impending bug attack, seem that much more unlikely.

The real weakness at the center of this production, though, is Ms. Judd, who confirms her inability to inhabit a part; the viewer is made constantly aware of how she’s “playing” someone desperate, “playing” dirty, and “playing” scared in the film’s later scenes. She fails to connect the dots, to portray Agnes as a believable woman rather than an outline of a dysfunctional woman moving from the first stage to the last. So we don’t understand her love for Peter — or her willful disregard for logic.

And without that, there’s nothing left; “Bug” is a character study without believable characters. More to the point, it’s a horror film without a single genuine scare.