Time Out says
It took playing an unrepentant murderer in Natural Born Killers for Woody Harrelson to shake off his Cheers comic-naf persona; given the incendiary turn the actor delivers in Oren Moverman's cop drama, he'll now need to take a legion of hayseed-buffoon parts to be viewed as something other than a walking human wound. Killers tapped the star's hidden vein of testosteronized violence, but Rampart hits the mother lode: His LAPD hotshot, Dave Brown, is the personification of white male rage. He drinks and smokes, he fucks and runs, he protects and serves his own interests while barely containing the urge to beat the shit out of everyone. (Brown's precinct nickname: "Date Rape.") This morally dubious officer is already in the internal-affairs crosshairs when he's caught on tape beating a motorist. People want Brown's head on a platter. Chances are he'll perform the decapitation himself.
Cowritten by Moverman and James Ellroy---a man who knows from sunbaked SoCal noir and damaged dudes---Rampart joins the ranks of films that posit L.A. as a lens-flared island of lost souls. But it's this paranoid, self-destructive Angeleno's personal hell that's the real backdrop, and Harrelson single-handedly takes this into The Shield-meets-Last Tango in Paris territory. With his bulletlike head and ramrod physique, the actor nails someone living and breathing a curdled culture of law-enforcement machismo; when he starts to come apart at the seams, you can feel the star plumbing emotional depths that he's only hinted at previously. The film has its narrative flaws and, occasionally, distracting stylistic flourishes. Harrelson's portrayal of a swinging dick staring down the abyss, however, is perilously close to perfect; it's the finest, most harrowing thing he's ever done.
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