Though his face is rounder and softer, there are definite reminders of his elder brother Ben in Casey Affleck’s pale, melancholy face. They are there in the voice, too, a Boston drawl that comes in a slightly higher register, gurgling a little as he swallows his words. It may be these traits that have caused many to brand him as an actor who has had an easy ride, having simply charged through doors that were opened for him by his lantern-jawed brother .

Soon, though, that will change. The junior Affleck has revealed what he is capable of, holding his own against Brad Pitt and Sam Shepard in the extraordinary, lyrical western The Assassination of Jesse James by the Coward Robert Ford.

Affleck plays Ford, the wannabe outlaw who became a friend of the legendary train robber and betrayed him to the authorities, finally shooting him in the back in his own home. Pitt is good as the murderous James, but the revelation is Affleck as Ford, a haunted individual who once made James his idol but found his life consumed by his act of treachery.

Given the storyline, it would be easy to draw parallels with the lives of the two brothers, but Affleck doesn’t look at it that way. “Ford never felt he was in Jesse James’s shadow,” he says. “James was someone he had hero-worshipped from a very early age, so when he met him he didn’t think, ‘I’m this guy’s peer and Ishould be getting the attention.’

“But I think maybe you’ve touched on something,” he concedes, “which is that deep down Ford thought he was capable of things that no one else saw him to be capable of. And that had less to do with Jesse James and more to do with the whole world.

“By all accounts he was an intelligent, cunning man. And relentless. After he killed Jesse James he was plagued by that one act his whole life. He’d set up a business and people would burn it down. He’d move to some place new, people would run him out of town. But he never gave up, he just kept going back and back. And with dignity. That’s something I admired about the character.”

Now 32, Affleck comes across as a thoughtful and educated personality, and his insights into Ford are perhaps what translates the director

Andrew Dominik’s understated adaptation of Ron Hansen’s novel into something so engaging. For one thing, he believes that the title alone says plenty about Ford’s vilification by a country that grew up on folk tales of Jesse James and saw him, mistakenly, as a romantic Wild West hero.

“When Jesse James gets killed people have a very strong reaction, so they have to make Ford out to be a two-dimensional villain. But he was obviously a complicated person. After all, he’d been asked by the state to kill James. How much of a villain could he be? How much of a coward could he be? He was a hero for a second, and then public sentiment turned against him very quickly. People hated him.”

Although he insists there’s no reflection of family politics in his performance as the underdog Ford, Affleck accepts that the film has a lot to say about fame, or infamy, a subject he became all too aware of when big brother Ben romanced Jennifer Lopez in the full glare of the media.

“It seems so obvious to me that the way people appear in the gossip papers is not often much like who they are,” he says. “Sometimes they are worse. I think, ‘Well, he’s an asshole, but he comes off pretty nice in the papers!’ But in the case of those I know well, who are super, super stars, what can you do but laugh at how they’re depicted contrasts with who they really are?”

But Affleck is intelligent enough to note that, “publicists and actors want to control their image. They have an idea of what makes them look good, so they present themselves in a bizarre two-dimensional way. Sometimes I think they’re right, because once you speak frankly, and are just who you are, it can come back to haunt you.” He smiles, apparently aware of the irony of the context in which he’s making this statement.

After ten years in New York, Affleck now calls Los Angeles home, and feels he’s adapted well to the urban sprawl. Still, it’s an odd situation for him to be in – to “go Hollywood”, having never pursued acting as teenager. “I studied acting in high school,” he says. “I was lucky. I had a teacher who encouraged me. Anybody who walked into his class wanted to be an actor, he just turned them on to the beauty of it. I owe everything to him.”

Affleck’s father, Tim, was an actor too, but since he left home while his two sons were still young, he doesn’t appear to have been much of an influence. Their mother Chris, a teacher, kept the family together, encouraging the kids to take roles as extras in TV ads, but Affleck insists their life wasn’t far out of the ordinary. “We’d run around in the street and get in trouble,” he shrugs. “There were a lot of kids on the street where I grew up. It was very interracial and it was working class.”

His roots seem important to Affleck, and he’ll next be seen playing the lead in the older Affleck’s directing debut Gone Baby Gone,a dark child-abduction drama set in Boston, where the pair grew up. However, in the wake of the Madeleine McCann case, this adaptation of Dennis Lehane’s 1998 novel was withdrawn from this year’s Times/bfi London Film Festival because of its sensitive subject matter, and may never be released in the UK.

A parent himself now, Affleck is expecting his second child with Summer Phoenix, the sister of the late River, whom he married last year after several years together. The couple already have a three-year-old son, Indiana. “It’s a sad story,” Affleck says of the McCann case. “It’s a parent’s worst nightmare. But I’m glad that they delayed the movie. It was the least that anyone could do.”

He adds that Ben was easy to work with, and very generous with advice, but points out that this only pertains to the film they made together, not, say, the string of Ocean’s films Casey has done, or the indie flick Lonesome Jim he did with Steve Buscemi. “Oh no,” he insists, “not about other films, just his own.” He laughs. “I would never take Ben’s terrible advice!”

The Assassination of Jesse James by the Coward Robert Ford is released on Nov 30

Explore Film

• Film Reviews

• Trailers

• DVD

• London Film Festival

Editor's choice

• The 100 Best Films of the Decade
• 70 facts you didn't know about Marvel
• The 50 Biggest Movies Of 2010