Robert Duvall's first foray into acting, writing and directing, 1997's "The Apostle," worked out so well that it inspired him to wear all three hats for a second time.

But the fact that "The Apostle" was so successful makes "Assassination Tango" even more of a disappointment.

"The Apostle" had a natural momentum, and Duvall — as a Texas preacher who moves away and reinvents himself after beating his wife's lover into a coma — had an irresistible charisma. He received nearly unanimous critical acclaim for the film and a best-actor Oscar nomination.

Six years later, Duvall seems to have lost all sense of pacing and tension. He lets some scenes here run too long and cuts away too quickly on others; and his fondness for improvisation yields wildly uneven results.

"Assassination Tango" meanders between thriller and love story but never truly succeeds as either. Sometimes it even feels like a documentary about the tango, with Duvall serving as filmmaker behind the camera and journalist in front of it. And that's probably what he should have made instead of this.

He stars as John J., a New York hit man who becomes fascinated with the tango, and with a dancer named Manuela (Duvall's longtime girlfriend, Luciana Pedraza), while on assignment in Argentina.

Duvall loves the dance in real life and wants to share that love with the world — hence the film. But "Assassination Tango" grinds to a halt during scenes like the one in which John sits in a club and interviews Manuela's aunt, a veteran dancer, about the history of tango: Who were the best dancers? What is the future of tango?

A more important question is, why should any of us pay attention?

The answer to that is the tango itself. The scenes in which Manuela and other dancers perform are sultry, sexy, beautifully choreographed. And Pedraza, in her film debut, has a naturally confident on-screen presence.

But Duvall, surprisingly, didn't craft a terribly compelling character for himself; John is actually a bit of a cipher. We know he's fiercely — almost obsessively — devoted to Jenny (Katherine Micheaux Miller), the young daughter of his girlfriend, Maggie (Kathy Baker), but we don't know why.

We know he views his job as just a job — he kills people for a living and feels no remorse — but we don't know how he got that way.

And the film's actual plot — in which he begins to suspect that he can't trust his contact in Buenos Aires (Ruben Blades) — becomes secondary to the tango and the tentative romance with Manuela, and does little to fill in the blanks.

Yet Duvall has the same tremendous presence he exhibits in every film. he's exhibited in every film he's appeared in over the past four decades. Like his preacher in "The Apostle," the hit man he plays here is a character full of contradictions. He's tough, wiry and unpredictable, yet an unabashed romantic who allows himself to be vulnerable and even funny at times.

At 72, though, Duvall continues to evolve both as an actor and a filmmaker. Surely another work as good as "The Apostle" is still in him.

The film is rated R for strong scenes of violence (including gunplay), occasional use of strong profanity and simulated sex. Running time: 114 minutes.