What Casey did next: the insider at Hollywood's dark heart

After starring in The Killer Inside Me, Casey Affleck made a documentary about Joaquin Phoenix. He talks fame, family and debauchery with James Mottram

Casey Affleck must be getting comfortable with controversy by now. Back in 2007 Gone Baby Gone, the Boston-set crime drama he made with his older brother Ben at the helm, was pulled from the London Film Festival because its plot bore a close resemblance to the kidnapping of Madeleine McCann. This summer Michael Winterbottom's Jim Thompson adaptation The Killer Inside Me, in which Affleck stars as a psychotic sheriff who brutally pulverises two women, has caused outrage. And now he's set to cause tongues to wag further with his directorial debut, I'm Still Here, a documentary about his brother-in-law, the twice Oscar-nominated actor Joaquin Phoenix.

According to the Los Angeles Times, when the film was shown to US buyers earlier in the year, it "overflowed" with Hollywood-style debauchery. Phoenix is seen snorting cocaine, enjoying oral sex with a publicist, ordering call girls and treating his assistants appallingly. It features lashings of male full-frontal nudity and even a "stomach-turning sequence in which someone feuding with Phoenix defecates on the actor while he's asleep". So how does it feel to be behind one of the most talked-about films of the moment? "I'm not sure that everyone is talking about the movie," Affleck sighs. "I think it might feel that way because it's such a teeny-tiny world, Hollywood. But I guess it's more pressure and it's more exposure than many first-time directors have."

In truth, the buzz around the film has been building gradually, after it became clear that the 34-year-old Affleck, who is married to Phoenix's younger sister Summer, had been filming his brother-in-law ever since he announced he was quitting acting to pursue a career as a hip-hop artist. Then came Phoenix's February 2009 appearance on David Letterman's show. Sporting a wild-man beard, sunglasses and a deathly complexion, reactions to his monosyllabic interview answers ranged from concern that he was heading the same way as his late brother River Phoenix, who died from a drugs overdose, to the notion that his behaviour – and Affleck's documentary – was an elaborate hoax.

"Well, it's not a hoax," Affleck tells me. "There's no hoax about it. I really wanted to do something that showed somebody that was very private and was also very, very, very well known. I wanted to do a super-intimate portrait of them. And I wanted to do something now because I knew that he was quitting acting to embark on this journey, this transition, to a music career – which he has been doing for a long time, by the way. It just seemed like it came out of the blue to everyone, because they just didn't know that about him. And I thought something interesting would happen over the course of that transition – and I was right."

The film is the result of two years of work, and a much longer friendship. "Joaquin and I have been very close friends since we were teenagers," says Affleck. They first met in 1995 on Affleck's big-screen debut, Gus Van Sant's To Die For, then became roommates and have remained "very close" ever since. It was Phoenix who introduced Affleck to his sister, who then became his girlfriend for six years before they married in 2003 (and since have had two sons, Indiana August, six, and Atticus, two). Given all this, Affleck remains one of the very few in a position to explain Phoenix's desire to call time on his acting.

The obvious line is that Phoenix, who had checked into rehab in the wake of playing Johnny Cash for the 2005 biopic Walk the Line, burnt out. Maybe he just felt he couldn't go any further with his career, I suggest to Affleck. "That's a good guess," he says. "It's a good guess for someone that doesn't know him. It certainly sounds plausible. I don't really think that that did it. Walk the Line, although it was what he became known for... I don't think the experience of making the film had an impact on him as much as people might think, just because he got nominated for an Oscar and everybody liked it. It was just one of those things."

If the film offers a unique insight into the warped nature of celebrity, Affleck's own career has certainly prepped him for it. Whether it is starring with A-listers such as George Clooney and Brad Pitt in Ocean's Eleven and its sequels, or seeing the media furore that surrounded his brother's called-off wedding to Jennifer Lopez, Affleck has been a bystander, watching the celebrity carousel spin them all ever faster. "People spend 20, 30 years of their life being not-famous, and then they get famous, and millions of strangers suddenly know them and it's very off-putting," he says. "I think everybody wants to keep the opportunities that fame affords you – the money and the work. But there aren't so many people that like the rest of it. I certainly don't."

Raised in a working-class suburb of Boston – his father a bartender, his mother a teacher – Affleck's blue-collar background has kept him grounded. When his brother became famous, winning an Oscar for co-writing 1997's Good Will Hunting, Affleck – who had a small role in it – stayed largely in his sibling's shadow. Less immediately handsome (he lacks his brother's Dan Dare-like jaw-line, and his blue eyes seem more sleepy), he plied his trade in little-seen arthouse movies like Gerry and Lonesome Jim. "I've never experienced anything but a modicum of success or total failure," he grins. "I don't expect much else when I go do a movie, to tell you the truth."

But all this changed in 2007, when he starred alongside Brad Pitt in Andrew Dominik's elegiac Western The Assassination of Jesse James by the Coward Robert Ford, which saw him nominated for an Oscar for Best Supporting Actor. Yet the film failed to set alight the box-office, and Affleck escaped further media scrutiny. "I think it was a shame that more people didn't get it," he laments. "It was kind of a disappointment... in terms of what people are seeing and what people like to see. It's a little depressing because I think it's an intensely beautiful movie with great performances – my own notwithstanding."

The Killer Inside Me is another matter. While Affleck's turn as Lou Ford is a masterclass in menace, his blank features chilling in their lack of pity, the film was widely deplored for its full-on scenes of violence, which included Affleck punching Jessica Alba into a coma and kicking Kate Hudson in the gut repeatedly. When the film premiered at the Sundance festival, one outraged female viewer stood up and yelled "How dare you?" at director Winterbottom. "I wasn't at Sundance but I heard the audio of it online," says Affleck. "It didn't sound like much – one person's thoughtless righteous indignation about violence in movies. It was not well articulated."

On release for over a month in the US, so far it's recouped a miserly $146,444 – no real surprise given the intensity of the violence, something that Affleck nevertheless argues was necessary. "The only way to depict this violence is to make it realistic," he says. "Otherwise you glamorise it. Movie after movie after movie shows violence in an ordinary way where the victims are dehumanised and people are jailed left and right and nobody cares about it. There's never blood. It's usually shown in a super-stylised, unrealistic way. I would prefer that all movies show violence in a realistic way. Because it's upsetting and it should be upsetting."

Next up is a possible collaboration with Ridley Scott on The Kind One, a 1930s-set drama in which he'd play an amnesiac who works for a mobster. Yet another violent crime film, following Gone Baby Gone and The Killer Inside Me, this is something of a coincidence, Affleck assures me. "The things that I want to do never come along. The thing is, you have to stay open. You might think 'What I want to do is a Jane Austen movie!' But they never come along! But if you think 'I want to do a crime novel', Pride and Prejudice will show up on your doorstep. It's just the way it is for me, and you've got to take the best of what's available and try to stay open to everything."

While that may be true, Affleck is not afraid to admit that he's quite choosy when it comes to his roles – a trait that sets him apart from his brother, who almost blew his career on a series of throwaway rom-coms. "I guess I am very selective," he nods. "Selective as I can be. It's hard sometimes. You say 'no' to a million things and then you're broke and you have to take something that might not be as good as some of the other stuff you said 'no' to." He lets out another sigh. "I wish that I could win the lottery and then I could sit back and work on only the things I love! But then so does everyone else in the world."

'The Killer Inside Me' is on general release. 'I'm Still Here' will be released later this year