If you don't recognize that African-American actor, there's a good reason: He's white. In Tropic Thunder, Robert Downey Jr. plays Kirk Lazarus, a very serious Oscar-winning actor cast in the most expensive Vietnam War film ever. Problem is, Lazarus's character, Sgt. Osiris, was originally written as black. So Lazarus decides to dye his skin and play Osiris, um, authentically. Funny? Sure. Dangerous? That's an understatement. ''If it's done right, it could be the type of role you called Peter Sellers to do 35 years ago,'' Downey says. ''If you don't do it right, we're going to hell.''