Casey Affleck has been trying for years to emerge from the shadow of his Oscar-winning big brother, Ben. First he tried making small independent movies, then he tried his hand at fashionable stage work and even wrote the script for an animated film. But now he has won recognition by shooting Brad Pitt in the back.

The scene takes place in his latest film, The Assassination of Jesse James by the Coward Robert Ford. “I think the clue to what happens is in the title,” says Affleck, dryly, and it won him an Oscar nomination for best supporting actor at this year’s awards.

Chronicling the legend of Jesse James, played by Pitt, and his eventual assassination by Ford, played by Affleck, 32, the film debunks the popular myth that the outlaw was a Robin Hood character robbing rich railway owners to help poor farmers.

“It was all nonsense,” says Affleck. “There is no evidence that he helped the poor. But James became a hero and Robert Ford was one of his followers. James was 34 and Ford only 19 when he shot him in 1881. On James’s tombstone it says, ‘In loving memory of my beloved son, murdered by a traitor and coward whose name is not worthy to appear here.’ After that, Ford was treated like a leper.”

How much did he know of the real story before filming began? “I knew quite a bit about Jesse James, like every schoolboy in America,” he says. “But nothing about Robert Ford. Many people claim to be great-grandsons or great-great-grandsons of Jesse James. Nobody claims to be related to Robert Ford. He was scorned for the last years of his life after he shot Jesse James.

“But he was not a coward to me,” Affleck insists. “He was a young guy, who possibly murdered James out of jealousy or to win a reward. The relationship became convoluted and complicated and he felt that he had to kill him.”

Is Affleck’s own relationship with brother Ben, 35, also complicated? Ben won an Oscar 10 years ago for co-writing Good Will Hunting with Matt Damon, and went on to appear in films such as Armageddon, Shakespeare in Love and Pearl Harbor. “We fought like hell as kids and we are both very competitive,” he says. “We also criticise each other. But when it comes down to it, we love each other and are very close and supportive. I have never been jealous, for one single second, of his success – and he has backed me all the way with my own.”

Affleck is also good friends with Damon and shared a flat with him five years ago in Soho, central London, while Damon, also a good friend of Ben, was doing a stint in West End theatre.

So does he also share his older brother’s taste in motors? “My first car was a 1969 Cadillac, a black four-door with tinted windows, which I bought for $3,000,” he recalls. “I drove that car everywhere and really enjoyed it. I must have made at least four trips across America, driving around 150,000 miles. That was at about 12 miles a gallon, though.” He feigns a look of self-disgust.

“Nowadays I think it is just a matter of trying to think more responsibly if you can,” he says, adding that, like his brother who recently confessed to feeling guilty about owning a Bentley, he prefers something a little more eco-friendly. “I have bought a Toyota Prius. I think a hybrid car is far better to have than a gas-guzzler.”

The two brothers grew up in Massachusetts in a household that he describes as “politically aware”. “I certainly never rated actors,” he says. “My heroes tended to be people like Martin Luther King and Gandhi.”

He admits to being something of an outsider as a child. “I begged my mother when I was about 10 to have plastic surgery to get pointy ears, so I could be an elf. How odd is that?” He was also more nerdy than actorish, studying physics at Columbia University in New York.

“I suddenly realised that all the other kids studying physics or astronomy were so much better than me,” he says. “I could never see me matching them. But I had started doing some acting work, as an extra, back home and thought that acting might be an easier option. So I started to get parts and realised it was something I could do naturally.” Today Affleck has a three-year-old son, with the exotic name of India August, with 29-year-old actress wife Summer Phoenix, sister of Joaquin. But he’s not yet revealed the name of his second son, born in January. “We are trying to keep his name a secret,” he says quirkily. “It will probably come out eventually but for the moment we have not made an announcement. I try to avoid talking about private matters.”

He must be losing sleep with such young children. “I have got used to getting by on three hours sleep a night,” he says. “It is good training for work on a film set, because you never really sleep well when you are working so hard.”

He is now driving his Prius around the canyons of the Hollywood Hills, attending auditions and meetings, having moved from New York about a year ago. “The biggest difference between the two cities is that the rats are beneath you in New York, in the sewers,” he says. “But they are right here alongside you in Hollywood.”

Welcome to my world, as his brother might say.

My stuff...

In my parking space A Toyota Prius, below

On my CD changer Old stuff such as Bob Dylan and Bruce Springsteen, plus some classical sounds such as Mozart and Vivaldi. That is to help put our sons to sleep when they are with us in the car

On my DVD player A whole range of stuff: The Elephant Man, The Good, the Bad and the Ugly, and The Great Escape

I would never throw away A photograph of my wife and kids. I travel light, hate to build up personal possessions and that picture is the only thing I take with me from home