When Ben Stiller phoned him a year or so ago, Downey wondered if he had finally met his match.
The role Stiller was offering Downey was an odd one.
It was for the action-comedy Tropic Thunder co-written and to be directed by Stiller.
Stiller was targeting Downey to play the character Kirk Lazarus, a roguish, hardcore, method-style Australian actor.
Mastering an Aussie accent was the easy part.
The great challenge for Downey was he not only had to play Aussie ocker Lazarus, but also play Lazarus portraying an African American army sergeant, Lincoln Osiris.
Confused? So was Downey.
"Yeah, sure I was," the actor laughed during an interview at a West Hollywood hotel recently.
Tropic Thunder is a spoof of Platoon, Full Metal Jacket, Hamburger Hill and other 1980s Hollywood films set during the Vietnam War.
The story follows a group of pampered, self-obsessed actors, including Lazarus, as they attempt to shoot a war movie in a jungle in South-East Asia.
The actors stumble into an area controlled by a drug syndicate, but are so self-absorbed they do not realise the heavily-armed traffickers who chopped off their director's head are the real deal.
They think they are other actors and it's all part of the movie.
Stiller, who also stars in Tropic Thunder, recruited a who's who of A-List Hollywood acting talent including Nick Nolte, Jack Black, Matthew McConaughey, Tobey Maguire, Jon Voight, Alicia Silverstone and a surprising cameo by a hard to recognise Tom Cruise, to act alongside Downey.
Stiller pokes fun at Hollywood in the film.
And he's in hot water with disability groups in the US for the repeated use of the word "retard" in one scene.
Downey also knew he was treading in sensitive territory as white Australian Lazarus playing African American Sgt Osiris.
"There were times I was a little freaked out where I thought `If I do this wrong it's kind of my fault'," Downey said.
Downey's Lazarus, who some critics believe is modelled on Russell Crowe, is a five-time Oscar winner and is so dedicated to pulling off a believable performance as Sgt Osiris he undergoes an operation to darken his skin.
Downey is hopeful African Americans will not be offended.
"If you see the movie, it's a non-issue," he said.
"If you read the script, initially you would say `This is probably a non-issue', but there's a lot of things that have to be managed and handled correctly down to the look and execution."
Tropic Thunder, dubbed by some American critics as the funniest film of 2008, will continue a golden year for Downey if it lives up to the hype.
Not too long ago Hollywood studios and directors deemed Downey a liability because of his drug history, but this year he clawed his way back to the top of the industry with the success of the action film, Iron Man.
In October, he begins Sherlock Holmes with director Guy Ritchie in London.
Seven years ago if Downey's photo was in a newspaper, it was likely one of his many police mug shots.
In April 2001, he was arrested when police found the actor wandering dazed in an alleyway after midnight in a seedy area of west Los Angeles.
He was arrested on suspicion of being under the influence of a controlled substance. A urine test detected Downey had cocaine in his system.
Five months earlier police found cocaine and methamphetamine in his Palm Springs hotel room.
Just three months before the Palm Springs bust, Downey had walked out of Corcoran state jail, located in the desert three hours north of Los Angeles.
An exasperated judge sent him to the jail for repeatedly violating probation for a 1996 arrest for drug possession, driving under the influence and carrying a concealed weapon.
Rehab and close friends, including Mel Gibson, who has had his own brushes with the law and an addiction problem, helped Downey through it.
The actor also found stability with his 2005 marriage to film producer, Susan Levin.
Downey says his life is now structured, with martial arts and tennis helping him control his dark side. He is enjoying being back at the top in Hollywood.
There's two Iron Man sequels in the works that will earn him a reported $US30 million ($NZ43 million)-plus and the feeling in Tinseltown is he will finally be rewarded with an Oscar in the next few years.
The actor is not getting carried away, however.
He knows the demons he left behind seven years ago are lurking nearby.
"Sometimes you have to be careful what you wish for."
