With Iron Man, in which he played a hard-partying playboy turned conscientious avenger, Downey proved that a cerebral character actor (in, admittedly, peak physical condition) could carry a superhero franchise even more convincingly than matinee idols like George Clooney and Hugh Jackman. So when Ben Stiller called him a year ago, asking him to put his suddenly sterling reputation on the line by donning blackface for the savage comedy Tropic Thunder, Downey was understandably conflicted.

“The movie is a minefield,” he says after a seemingly interminable moment of reflection. “A year ago, I was just thinking about me, and I was thinking about Iron Man. My first reaction was, ‘F**k Ben Stiller.’ He wanted to do a big movie with me assuming the highest risk factor, where I might be subjected to ridicule and hated for doing something that I knew was wrong in the first place. What happened to just being an actor for hire?”

In Tropic Thunder, Downey plays Kirk Lazarus, an award-winning actor so taken by his press clippings that he elects to play an African-American soldier in the Vietnam War. Never mind that the character is Australian, with pale skin, bleached-blond hair and brilliant blue eyes – a choice possibly inspired by Russell Crowe, though neither Downey nor Stiller, who wrote and directed, will say. Lazarus is a method actor so insanely dedicated to his craft that he darkens his complexion and refuses to break character even after the cameras stop rolling.

“The whole premise is so ridiculous that it almost makes sense,” Downey says with a wry smile. “I loved playing a guy who believes that he is the greatest actor in the world, someone so utterly crazy and self-important that he thinks he can portray the black experience better than a black actor.”

While he was initially hesitant to become the most politically incorrect component of Stiller’s vicious satire, which Downey describes as “entertainment set up by people high-minded enough not to be racist and offensive,” he savored the opportunity to poke Hollywood in the eye. Not that he’s trying to bite the hand that feeds – Downey, who will play the world’s most celebrated detective in Guy Ritchie’s upcoming Sherlock Holmes, admits that Iron Man has left him flush with leading roles and lucrative business opportunities.

Still, he recognizes the absurdities inherent in his craft and in Lazarus.

“The movie is based on the belief that what actors do is disgusting, pathetic and needy,” he says. “That’s certainly true at times, but not always. In my character’s case, it’s a question of how far-reaching can someone’s narcissism go, and how much can that narcissism be coddled by the people around him.

“I’ve worked with actors who needed to dress up like animals to get in character, who told me with a straight face, ‘You need to wear this reindeer mask, and I need to wear this bear mask, and then we can rehearse.’ Well, no, we don’t. Who teaches this garbage? Maybe it works for them, but I wouldn’t know. I’m not an artiste. I can’t be bothered."