Actor Joaquin Phoenix almost fell out with his brother-in-law and best friend Casey Affleck while shooting spoof documentary I'm Still Here - because the Oscar nominee was desperate to let his worried pals know his public meltdown wasn't for real.

The Walk The Line star shocked fans in late 2008 after announcing he was quitting Hollywood and launching himself as a rapper. He grew a beard and adopted a scruffy look as he promoted what he claimed was his final film, Two Lovers, but sparked fears for his mental wellbeing following a bizarre mumbling appearance on U.S. talk show the Late Show with David Letterman.

He maintained the hoax for months, and it wasn't until September, 2009, that he and director Affleck came clean about the whole stunt, revealing that it was all part of their mockumentary.

However, Phoenix admits he found it tough keeping up appearances as his friends and family feared he was genuinely losing his mind - but Affleck forbid him from sharing their secret with those outside the project.

He tells Interview magazine, "It was really hard working with someone who is that good of a friend. He's my best friend. I respect him. I think he's an amazing and talented actor and filmmaker. It was my dream since we were young to work with Casey. But it was very difficult because Casey was really intent on keeping it secret and I, of course, was a bit of a p**sy at times, where I was like, 'Well, I would like to tell my friend who has been calling me for a f**king month, 'Don't worry, everything is cool.' So we had arguments about that.

"Initially, it was just basically supposed to be a bad, glorified Saturday Night Live skit. But Casey was really intent on me getting out there publicly and humiliating myself as much as possible.

"Casey loves uncomfortable humour, awkward humour. We wanted to capture that random moment of like, 'Oh, god. Please. I'm not watching this.' So he was really pushing. There were a couple things where I was like, 'Nope. I'm not going to Vegas and getting on a stage. I'm not f**king doing it.' And you know, we did it and I was like, 'Dude, I have to do these interviews and I'm keeping this s**t going.'"