It might sound odd to hear that “The Assassination of Jesse James by the Coward Robert Ford” is not a Western. It’s a movie set in the American West, about familiar legends from a legendary time and place. There is a holdup scene, horses and shootouts. But it is incompatible with, if not incomparable to, that genre’s familiar good guys and bad guys, tough speeches, quick draws and fancy gunplay. The few gunfights are unnervingly realistic and awkward. So are the people.

Writer and director Andrew Dominik portrays the much-reviled Robert Ford (Casey Affleck), who reputedly shot Jesse James (Brad Pitt) in the back as he was dusting off a picture in his home, as a young man who’s had his innocence mocked and brow-beaten out of him. In the film, Ford, 19, admires Jesse so much he keeps a box of books about him under his bed. Although there isn’t much action in this longish story, it’s fascinating to watch Ford change from a scheming worshipper into a reluctant schemer, who isn’t sure by the end why he’s done what he’s done, or what it means.

James, a hard man suffering from severe stress-induced paranoia, isn’t any more easily pegged. Yet when he hands Ford a new nickel-plated sidearm, the two seem like refugees from another, softer movie. Overwhelmed with gratefulness, Ford grins like a proud child at the gift given to him by his hero, whom he’s planning to kill. Later, a girlfriend (Zooey Deschanel) asks Ford why he did it. “He was going to kill me,” Ford says, “and for the money.” There are other reasons, too, none any more final. What’s great about “Assassination” is that it maintains this shiftiness to the end, like a picture that alters when you turn it in the light. The verdicts in the title and narration might be cut and dried, but the movie leaves the truth as wide-open as the landscape. (R)