Rampart (15)

Here's an idea for a movie: the story of an LA cop who doesn't take backhanders, doesn't plant evidence, doesn't beat up suspects, doesn't spout racist abuse, and doesn't flagrantly disregard his force's proud motto "To Protect and Serve".

He is an untested type, and will not be coming to a theatre near you very soon. Movies about his polar opposite, however, will never be in short supply – think Richard Gere in Internal Affairs, James Cromwell in L.A. Confidential, Denzel Washington in Training Day, Kurt Russell in Dark Blue. Et multi alii. To this roll of dishonour we can now add Woody Harrelson, whose dirty cop in Rampart seems to be contending for the biggest sleazeball on the beat. It's a crowded market out there.

The film is written, almost inevitably, by James Ellroy, connoisseur of all things rotten in the City of Angels. His co-writer, and the film's director, is Oren Moverman, whose brilliant debut, The Messenger (out in the UK last year), also starred Harrelson as an army officer deputed to "casualty notification" – bringing the bad news to the relatives of soldiers killed on active duty. If that film was about coping with a difficult job, Rampart is all about not coping, about making a difficult job much worse. Harrelson plays LA patrol cop Dave "Date Rape" Brown, whose unfortunate moniker alludes to a notorious case in which he may have murdered a serial date rapist.

It is by no means the only black spot on his record. The trouble with Dave is that he regards police work as a licence to do pretty much as he likes. "This used to be a glorious soldiers' department," he says of the old days, but now it's 1999, and in the wake of "Rampart" (an actual scandal that exposed deep-lying corruption in the LAPD gang units) the force is under renewed scrutiny: dinosaurs such as Dave are on the way out.

His personal life is also in disarray, what with two semi-detached ex-wives who happen to be sisters (Cynthia Nixon, Anne Heche) and a sulky teenage daughter who greets him, "Hey, Date Rape." It's not a happy household, so when Dave isn't popping pills, gargling scotch or cheating on his taxes he's picking up women in singles bars.

Moverman takes his time sketching in the shape of Dave's life and the sun-blasted streets on which it's played out. His camera focuses intently on Harrelson's tanned head, as bald as a snake's, seen through the moving window of his police cruiser.

Moverman's shooting style is faintly Altmanesque, favouring zooms and overlapping dialogue; in one scene he keeps panning left-to-right in a three-way conversation, possibly to suggest Dave's mental disorientation under the influence of booze and uppers – or possibly just for the hell of it. As a study in character it feels incisive and detailed, though you start to wonder if it isn't rather plot-starved. Then something does happen: Dave metes out a Rodney King-style beating to a miscreant who happened to rear-end his car. The incident is caught on video, and suddenly Dave's mug is smeared all over the TV news: another PR disaster for the force. He's hauled before the district attorney (Sigourney Weaver) who asks him, "Have you thought about retirement?"

It's a fair question, but Dave isn't the type to go quietly. The picture starts to crowd with plot prompts. A woman he picks up, Linda (Robin Wright), turns out to be a defence attorney who could help him. Strapped for cash, he asks an old cop friend (Ned Beatty) to help him with a loan but gets unwanted advice instead: "You could just stop beating people up."

A wheelchair-bound vagrant (Ben Foster, Harrelson's co-star in The Messenger) may have witnessed Dave shooting a man during a stick-up – will he try to blackmail him? With several directions for the plot to take, we wait to see which one it will pursue. And, astonishingly, it chooses none of them. The film scuttles sideways, and in circles: but it refuses to go forward. It's as if Moverman and Ellroy have put the stew in the pot but won't turn on the oven.

Did they become mesmerised by Harrelson's performance? It is a powerhouse turn – he's in every scene – showing not just the brutality and cunning of this professional thug but also qualities that people respond to, such as his affability and insinuating charisma. (He also indulges in some fancy wordplay, courtesy of Ellroy one imagines, that doesn't quite click.)

Harrelson holds the thing together, and plays off the others potently. His verbal jousts with authority figures Sigourney Weaver (who's terrific), Steve Buscemi and Ice Cube are riveting; it's as though Dave raises his game whenever he's in danger of being cornered. To Ice Cube's Internal Affairs investigator he says, "I'm not a racist. I hate all people equally."

Scene by scene the picture works, stumbling only on a movie cliché of the psychologically damaged when Dave calls at an underground sex club (Michael Fassbender tried the same thing in Shame) and scoffs a greasy taco – a frightening spectacle in itself.

Thinking about it now, The Messenger didn't contain much of a plot either, but it didn't need a motor to sustain its series of agonising encounters. Rampart is different, because it assembles the elements of a thriller and then doesn't bother to resolve them. It frustrates the more for having so many scenes that are pointed and well-written – and go nowhere. Perhaps that's just Moverman's métier, to write great scenes and hire brilliant actors to play them. It's more than most directors are capable of. But in the end this is like Dave's police work: lots of energy, not much discipline.