Will acting ruin Vin Diesel? Quite possibly, judging from "A Man Apart," an alleged action drama in which he stars and actually tries to deliver a real performance. In his previous outing, "XXX," a Joe-Sixpack-as-James-Bond adventure for the extreme-sports-and-tattoos demographic, Diesel was squarely in his element, playing an overly muscled adrenaline addict with a good heart and a perpetual wry smile on an otherwise stony face. In short, a role that required as much effort from him as slipping on a form-fitting T-shirt. More than anything else he seemed to be just having fun, and it gave the movie a goofy, infectious vitality.

In "A Man Apart," Diesel plays Sean Vetter, a dour, up-from-L.A.-mean-streets DEA agent whose beloved wife is murdered by drug dealers, and who must wade deeper and deeper into a sewer of moral ambiguity to avenge her. The potentially engaging role calls for some profound sense of loss to justify his increasingly questionable tactics -- and an awareness that he may just be turning into the very kind of human garbage he's pursuing.

Diesel tries -- boy, does he -- but he can't deliver the goods. Then again, neither can director F. Gary Gray, nor can screenwriters Christian Gudegast and Paul Scheuring. For all its splattery shootouts and punch-outs and its flirtation with amorality, "A Man Apart" is astonishingly leaden on almost all fronts. It really should be arrested for impersonating an interesting movie.

"A Man Apart" has the usual share of guy-flick cliches -- an alienated loner struggling against really evil bad guys as well as a constraining system. However, action -- executed with some kind of style or flair -- is what you trade your bucks for, and "A Man Apart" starts out well enough, with Vetter & Co. busting a cartel leader in a slam-bang opening scene. The ensuing power vacuum -- or rather, a new kingpin's bloody rush to fill it -- promises more such scenes.

Gray and his writers have a knack for lowbrow, deadpan wit. For instance, one of Vetter's old friends from the street, a fat dude known as Big Sexy, announces he has the best and most ruthless drug-sniffing dog in L.A. -- which turns out to be a Chihuahua, making its entrance with an ominous buildup worthy of a pit bull. Occasional patches of dialogue crack you up with foulmouthed understatements.

Unfortunately, though, most of the movie is devoted to emphasizing Vetter's existential angst. Statically. Following just about every scene of plot advancement come several lingering scenes of Vetter (a) contemplating the ocean or (b) thinking of his dead wife or (c) contemplating the ocean and thinking of his dead wife or d) looking meaningfully into some form of nothingness, possibly because he can't get to the ocean. You can do a lot of things in an action thriller, as long as you're not boring or tedious. Gray manages both.

The same can be said of Diesel's acting. The former professional bouncer seems to want to show complicated emotions, but it's as if all his machismo gets in the way. He ends up looking chronically sensitive, or peculiarly confused. When, for example, the kingpin Vetter has put in jail tells him that he must become a monster in order to stop the new monster taking over the cartel, Diesel doesn't look any more distressed than if he's been told his shoes don't match his pants.

The screenplay further deflates any dramatic tension it might have generated: Never is Vetter subsequently put in a situation that forces him to question the morality of what he's doing. Every time he pulls a trigger or swings a fist, he suffers no qualms or hesitancy. Maybe all that salty air from the ocean washes away a man's doubts.

A Man Apart (115 minutes, at area theaters) is rated R for violence, language, sexuality and nudity.

Vin Diesel (with Jacqueline Obradors) tries gamely to exercise what he may not have -- acting chops.