The Assassination of Jesse James by the Coward Robert Ford (15)

If cinema was only about an ability to deliver striking images of landscape, of weather, of human beings caught in their time then Andrew Dominik's long, lyrical account of the twilight months of Jesse James and his gang would be held in very high regard.

Photographed in a medley of browns, ochres and golds by Coen Brothers regular Roger Deakins, it has the look of a daguerreotype in which figures behind the glass frame seem to shrug off their pictorial stiffness and breathe the air of 1880s Missouri and Kansas. Sometimes the edges of the screen blur, as if we are watching events unfold through the flawed, rippled glass of a 19th-century window.

It is a haunting film to look at. This intense visual concentration, however, comes at the expense of nearly everything else: pace, excitement, humour, contrast, depth. Dominik is adapting from Ron Hansen's novel of the same name, and filters much of it through a narrative voiceover that reaches for poetry ("insomnia stained his eyes like soot") yet sounds rather too mannered for its own good.

And, given how much time Dominik has been allowed it runs 20 minutes shy of three hours he doesn't radically alter the mythic revisionist West we already know through Robert Altman, Walter Hill and most notably Clint Eastwood's Unforgiven. Gunfights are clumsy, banal affairs in which most shots are wildly off target, while the one major train robbery at the start of the film is a brutal farce quite different from the elegantly planned heists we used to see in Westerns.

The giveaway of that unwieldy title hides the fact that the film is really about hero worship, and its dark flip-side. When the 19-year-old Robert Ford (Casey Affleck) joins the James gang, he first tries to ingratiate himself with Frank James (Sam Shepard), who cruelly rebuffs him ("You don't have the ingredients, son"). But Ford has better luck with the more celebrated brother Jesse (Brad Pitt), who lets him hang around rather like Dr Johnson did with Boswell he's a bit of a pest, but the adulation is pleasant background noise.

The young man has in fact idolised Jesse for years, collecting storybooks about the outlaw and memorising personal details such as his eye colour and shoe size. His devotion, which becomes a joke among the gang, gradually curdles into impersonation. Ford begins the film as a ragamuffin in a shabby Artful Dodger hat; by the end he's wearing Jesse's signature look of white shirt and black waistcoat. As Jesse asks him at one point: "Are you gonna be like me, or are you gonna be me?"

This two-handed struggle between the outlaw and his protg is interspersed with a chronicle of what befell the other members of the gang, some of the loudest-laughing but least interesting desperadoes ever to occupy the screen. One bit of score-settling Jesse visits on a treacherous associate requires about 20 minutes of tensionless preparation.

In the absence of forward momentum, we turn to the acting for diversion. While Pitt looks terrific (he nearly always does) his fiddly performance cigar-puffing, teeth-sucking, thousand-yard-staring doesn't translate into inner complexity. The wordy narrative keeps assuring us that Jesse is a man of moods, cussed and unpredictable, but we get no clue as to what's driving him, or what he loves (Mary-Louise Parker as his wife is given an insultingly marginal role, and Shepard disappears altogether from the picture halfway through).

In one scene, Jesse drags a kid into a barn and beats him up; a few minutes later we see Jesse weeping by his horse. But there's no telling why he passes so suddenly from violence to lachrymosity.

Pitt's preening seems doubly inadequate set against the film's one great performance. Apart from his footslog through Gus Van Sant's Gerry, I had not really noticed Affleck before, but his work here makes me wish I had. There is a quality of stillness in his gaze that feels absolutely right for this watchful fellow, part hero-worshipper, part stalker. Rather than the "coward" the film's title brands him with, Affleck makes Robert Ford a sensitive, unstable kid who couldn't handle his own ambition. "I been a nobody all my life," he says, and the effort of becoming a somebody is almost too painful to witness. (The echoes of Mark Chapman and John Lennon are presumably intentional.)

The last 20 minutes of the film are a coda on what became of Ford and his older brother Charley (Sam Rockwell), acting out their own notoriety the first time as tragedy, the second time as bleak, soul-scorching farce. Indeed, Robert Ford's fate is in a way more interesting than the story of his hero, but by the time Dominik gets round to it the movie is dead and buried or ought to be.

Compared to Walter Hill's 1980 movie The Long Riders, also about the James gang, this feels an underpowered affair. Indeed, compared to Dominik's own previous movie, Chopper (2000), it is impossibly meek. Career criminal Mark "Chopper" Read also harboured psychopathic tendencies, kept a close eye on his legend, and was eventually traduced by his own associates. Eric Bana worked up a blistering performance that said little about crime but a great deal about male loneliness and rage.

Chopper will never be very famous outside of Australia, but his floridly violent personality made an impression on film that most outlaws would kill for. One suspects that Jesse James would have been first among them.