AFTER a walk on the dark side with the Oscar-winning No Country For Old Men, filmmaking siblings Joel and Ethan Coen return to the screwball side of the street. The themes remain the same – greed and bad decisions – but it's understandable that, after the grimness of adapting the godless gothic of No Country For Old Men, this time they want to get playful.

Things are set in motion when CIA analyst Osbourne 'Ozzie' Cox (John Malkovich) is called into a meeting and unceremoniously sacked. Instead of accepting the humiliation of a security downgrade, he ups and quits. He breaks the news to his wife Katie (Tilda Swinton, reviving some of her ice maiden frost from Michael Clayton), but assures her that he plans to write his memoirs. "Who'd read that?" she snipes.

The news only makes Katie proceed with her plan to divorce the pompous Osbourne and run away with her married US marshal Harry Pfarrer (George Clooney trying way too hard), who is proud that he's never shot his gun, yet as a serial philanderer is constantly discharging his weapon.

Harry is the brothers' main target for their character-defining fussiness. In fact, he is given a whole box of tics, including a mania for flooring, a brief glimpse of his self-invented anatomically implausible sex toy, and the fact that after every adulterous liaison he announces "maybe I have time for a run", as if the coitus is part of his workout routine.

These characters and others are brought into conflict when Ozzie Cox's disc containing his manuscript ends up in the possession of two idiotic health-club employees, Chad Feldheimer (Brad Pitt) and Linda Litzke (Frances McDormand). Chad is enchanted by the information but Linda sees it as her chance to get the cosmetic surgery she feels will help her get on in life ("I have limited breasts and a ginormous ass"). Linda needs money because she's pathologically convinced this is no country for old women, either. In one of the film's more graceful moments, she walks through a park looking for the internet date she arranged to meet while online – past bench after bench of men waiting for their own Linda.

For a while, Burn After Reading promises to be reasonably engaging in an aimless way. When JK Simmons' exasperated special agent says of the story, "Report back to me when it makes sense," he's summarising this archetypal Coen venture as well as anyone can. It moves slowly and steadily, but it's never exactly dull, just mild. Its chief pleasure for half the time is Brad Pitt as a mouth-breathing gym bunny with streaked blond hair and a bubble for a brain. "I am a mere Good Samaritan," he insists as he extorts a cursing Malkovich, but it's the physical side of his comedy that makes his performance a delight, whether grooving to unheard music, scowling at having to wear a suit like a kid at Sunday school or turning his eyes into cartoon slits of calculating cunning or wide and untroubled by thought of any kind. No one else in the films is as transparent, or fun.

The Ladykillers, Intolerable Cruelty and The Hudsucker Proxy all suggest that the smartass comedy is swampy ground for the brothers, so it's not surprising that Burn After Reading is a messy affair with flat longueurs, some nice asides and a fatal indecisiveness about whether it wants to savage vanity, stupidity or selfishness. According to the Coens, they cast the picture around people they had enjoyed working with in previous movies (Clooney and McDormand, Joel Coen's wife). They also wrote roles specifically for people they had long wanted to work with (Pitt, Swinton and Malkovich). This cast-first then write-around approach may explain why the performances and relationships feel so disjointed and why the film is like a shapeless anecdote told to you by a very old, absent-minded person on a late bus. A story where people get punched, shot, burned and killed.

As with No Country, the ending happens off-screen, but what seemed low-key and appropriate in No Country seems sourly misanthropic and forgetful in Burn After Reading. In the end it's as throwaway as the title suggests.

• On general release from Friday

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