Without twisting anyone’s neck through 360 degrees, the man who gave us The Exorcist messes with heads in other ways in this effective, if increasingly hysterical, psychological thriller.

Adapted by Tracy Letts from his own stage play, Bug puts a nervy victim of spousal abuse in bed with a paranoid delusional ex-soldier (with schizophrenic tendencies, naturally) and lights the blue touchpaper. Or rather, yellow flypaper.

After a night slinging drinks at the local lesbian bar, Agnes (Judd) is persuaded by her friend R.C. (Lynn Collins, The Number 23) to bring the unassuming Peter (Shannon) back to her grotty motel apartment.

Since Peter has no place to go, Agnes lets him stay the night. Next morning, the peace is shattered by the unwelcome return of her recently paroled ex-husband, Jerry (the impressively mean Connick). His visit is short but sharp.

Left in a fragile state of mind, Agnes takes Peter to bed. She wakes up to find him scouring the sheets for bugs. He’s been bitten. Can’t she see them? There. Where? Ah, there they are. And soon they’re everywhere…

With a limited budget and the action largely confined to one set, Friedkin allows Bug to pupate with jittery camerawork and the disconcertingly insistent use of sound; phones trill, crickets chirrup, helicopters hover overhead…

But what makes it especially creepy is that it’s played with deadly seriousness. Judd and Shannon go doo-lally with total and admirable conviction, embracing their paranoia with near-orgasmic zeal. Theirs is one very twisted love story.

Their descent into madness is, however, rather abrupt. It’s as though Letts’ initially measured script is itself bitten in its mid-section and goes into anaphylactic shock.

The drama swells alarmingly, and several areas are sorely affected by a nasty rush of blood. (The doctor’s house-call doesn’t end well and Peter’s crazed bout of auto-dentistry - sans anaesthetic – is certainly not for the faint-hearted.)

It’s also clear that there’s something odd about Peter from the outset - he freely admits that “women aren’t my thing”. So it’s doubtful that two apparently rational women would consider even chatting to him, let alone invite him home for drinks.

But the buzzword here is ‘unreal’, and though it’s not the most subtle dig at the US military machine you’ll ever see, Bug is an itchy, twitchy little blighter that really gets under your skin.

Elliott Noble