Assassination Tango is a flaky, delectable lark. It's the second feature that Robert Duvall has written, directed, and starred in, and, like the first, the magnificent backwater Christian psychodrama ''The Apostle'' (1997), it offers him the chance to play an aging sly dog who is jovial and loyal and charming on the surface but also, deep down, a violent killer. Duvall, it's clear, relishes the contradiction. He has played more than his share of tough nuts and tyrants (the macho Marine dad in ''The Great Santini,'' the wild-eyed war junkie in ''Apocalypse Now,'' Stalin and Eichmann), the result of which is that his career, with its alternating currents of chivalry and ruthlessness, has come to seem an ongoing confessional study of masculine power in all of its shades: jaunty, noble, tormented. Acting doesn't get more personal, or much greater.
In ''Assassination Tango,'' Duvall wears long, thin, graying hair plastered back into a bun. With his mustache and elegantly craggy face, he looks courtly yet threatening, like a senior member of the Hell's Angels. Duvall is John J., aging lord of the Brooklyn streets, who enjoys a complicated double life. At the local dance hall, he's an enthusiastic amateur hoofer, and he is also utterly, touchingly devoted to his hairdresser girlfriend, Maggie (Kathy Baker), and to her young daughter (Katherine Micheaux Miller), whom he dotes on more protectively than if she were his own. They're the family he never had, yet they have no idea that this hipster patriarch, so loving and light on his feet, is also a freelance hitman who regularly slips into the night, gun and silencer in hand.
Duvall, letting his scenes play out with a wonderful unfussy realism, dives into the plucky fascination of John J.'s dual nature, giving him a shockingly short fuse as well as hints of a past littered with lost romance. Duvall also reveals a new vulnerability about his own aging, as in the terrific scene in which John J. harangues a cop for having observed that he looks ''tired.'' Ordered by the local Mob boss to fly down to Buenos Aires, John J. is given the task of assassinating an old fascist general. When he learns that the hit must be delayed for several weeks, he passes the time haunting the city's dance parlors, where he becomes possessed by the angular erotic grace of the Argentinean tango, and by a lovely dancer (Luciana Pedraza) who agrees to school him in it.
It's obvious that Duvall made John J. into a dancer because... well, he wanted to do a movie about the tango, one that could feature Pedraza, his real-life girlfriend, who has an alluring inscrutability. In the hands of an actor (or filmmaker) less supple, this impulse might have come off as a thin indulgence, yet in ''Assassination Tango'' it results in one of the most spry and irresistible character studies of Duvall's career. It's arresting to see John J. plan and execute the hit, in what turns into a portrait of justifiable paranoia, and his ease on the dance floor seems to emerge right out of his wary, improvisatory cat burglar's cunning. The dance scenes are lovely, but by the end it hits you that the assassination is the movie's real tango, and that few artists have caught the swirl and kick of modern male aggression better than Duvall.