By now you probably know that "Tropic Thunder" stars some of Hollywood's biggest stars — from Jack Black to Ben Stiller, from Tom Cruise to Tobey Maguire — all willing to send up their own images for a laugh. You may also have heard that none of them is being talked about with as much acclaim as Robert Downey Jr., who plays Kirk Lazarus, a multiple-Oscar winner so committed to his craft that he literally transforms into a black man to portray a Vietnam solder in the film within the film. His performance is different, you hear, satirical and all in good fun. But you're worried, of course, because Downey isn't the first white man to play an African-American in recent cinema, and those portrayals don't always turn out so well.

But did you know that he's not even the first person in his own family to take on such a role? In crafting Lazarus, Downey says he owes a lot to his father, who directed a blaxploitation film nearly 40 years ago called "Putney Swope," about a black ad executive who ends up running the entire agency. Because Arnold Johnson, the actor portraying Swope, had trouble remembering his lines, Robert Downey Sr. dubbed his own voice over Johnson's during post-production.

Downey, who would have been about 3 or 4 years old when his father made "Swope," says that Lazarus' deep voice came from his vivid memories of that film.

"I started remembering my dad did this movie called 'Putney Swope.' I was in New York [at that time] and the character was supposed to be a Vietnam soldier, so I was remembering some of the folks who were hanging out in the West Village back then," Downey said. "Without it being too specific, I just started this gravelly, cool, very world-weary voice, and I said, 'Oh my God, I'm going to have a ball with this.'

"I laid the voice on [Stiller] and he laughed, and I thought, you know, he's just laughing to make me feel OK, and then he's going to change it all when we get there," Downey added. "And then I got there [and nothing was changed]."

Stiller, who co-wrote and directed the film, says the deep baritone fit the character perfectly. "He was really improvising a lot of the time, and there was a little bit of a question like, 'Is that intelligible? Can we hear that?' " Stiller recalled. "We did some looping too, but it really was who the character was and what he was playing. The idea of this Australian guy playing this black guy and his interpretation of that — that was always what we were satirizing, the idea of actors making wrong-headed moves to try to be taken seriously."

In the end, it's somewhat fitting that Lazarus' voice came from memories of Downey's own youth, because everything old is new again for the actor, who was nominated for an Oscar in the early '90s (for his portrayal of Charlie Chaplin) but was sidetracked professionally by debilitating addictions for much of the next decade. Thanks to the double whammy of "Iron Man" and "Tropic Thunder," Downey once again finds himself high on the Hollywood food chain. But while he drew raves for his unconventional portrayal of Tony Stark, it's for his role as Lazarus that many are already pegging him as an early favorite for Best Supporting Actor at next year's Academy Awards.

"I think this is going to be a big coming-out party for him as a comedian," co-star Jack Black said. "You knew he was a great actor with 'Chaplin,' [but] all of the films he's made were great dramatic films. This is the first one where it's been full-blown comedy, and his skills are sharp."

Is that why, despite initial misgivings, he ultimately took the role? To showcase his comedic side?

"I don't know," Downey said. "Life is too goddamn serious. Things are scary! I got to laugh my ass off [in this movie]. I'll take that over rocket science any day. And I'll certainly take that over self-important dramatist."

"Tropic Thunder" opens Wednesday (August 13).

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