'Thunder' clouds form for busy thesp

Robert Downey Jr. has always been quite the chameleon, shifting into characters with an incredible ease.

For his supporting turn in “Tropic Thunder” as Aussie actor Kirk Lazarus, who morphs quite literally into an African-American soldier during a tour of duty in Vietnam for a big-budget biopic, Downey had a transformation unlike any of his other roles.

Downey nabbed the role of Lazarus in the Ben Stiller-helmed mockumentary (essentially a film within a film within a film) while on vacation with his wife in Hawaii.

“I was in pre-production for ‘Iron Man,’ he recalls of the serendipitous casting, “and a friend of mine said, ‘Hey, Stiller’s in town; you might want to give him a call.’ ”

That call turned into dinner, dinner turned into a follow-up phone call, and next thing Downey knows, he’s back in Hawaii on location for the approximate six-month shoot.

Though slightly intrepid about tackling a role that, given both America’s wartime atmosphere and the racial passing of Lazarus, could clearly court controversy, Downey decided to nevertheless take the leap.

“I made sure that it wasn’t offensive,” says Downey of his mercilessly comedic performance, “and that we just sort of rode that razor’s edge. I thought how strange it was that to get into the comedy king’s club, I’d have to be tarred and feathered and put in special effects makeup throughout much of the film.

“But when I thought about how Jack Black was going to be strapped naked to an ass and tripping on dope the entire time, it suddenly all felt OK. Besides, everyone who read the script thought it was the funniest thing they’d ever read.”

Downey’s main inspiration in prepping for “Thunder” came from his father, Robert Downey Sr., who in 1968 wrote and directed the satirical racial commentary “Putney Swope.”

“I conjured up a sort of Putneyesque voice,” explains Downey of how he birthed Lazarus into being. “It kind of alarmed me, because as soon as I had this voice, I felt so free, and I felt like nothing I could say could sound wrong. There was just some sort of transcendence to the role. It felt like it was sort of closing this circle of legacy between my dad and I. To have my dad watch the film, half-falling out of his seat, meant the world to me.”

Favorite film this year

“What year are we in? I’m a year behind. Ask me next year when I’ve had a break.”

Best advice

“Mom told me that ‘The truth is the truth.’ That one took about a decade to sink in, but it had a big payoff, let me tell you.”

Inspiration

“I would say as much as anything it’s the company that I get to keep. Ben Stiller really raised the bar very high and then really supported me when I said I can even put that up an inch or two higher.”

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