Playing a dog-tired booze-hound cop who is surprised to find a spark left in his conscience, Bruce Willis gives his best performance in years — his acting has a lived-in authenticity that pairs up beautifully with his still-potent power. He almost makes you forgive the plot contrivances that stretch belief to the breaking point. Willis’ NYPD detective Jack Mosely, with a rep for cutting corners, is nursing a hangover when he’s thrown a quickie job: Drive petty thief Eddie Bunker (Mos Def) sixteen blocks from his holding cell to the grand jury. Making an unscheduled stop for scotch in Chinatown, Jack finds that the bad guys don’t want Eddie to testify. The bigger surprise is that the bad guys are cops, led by Frank Nugent (a vividly smarmy David Morse), Jack’s former partner. Director Richard Donner (Lethal Weapon) keeps the action crackling. And Willis and Def — funny even if he overdoes the whiny shrieking — are a terrific team. Until Richard Wenk’s script drives the characters into a brick wall of pukey sentiment, it’s a wild ride.