By Adam B. Vary

If you don't recognize that African-American actor standing between Jack Black and Ben Stiller, there's a good reason: He's white. In Tropic Thunder, an epic action comedy co-written and directed by Stiller, 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.''

The film marks Stiller's first directing effort since 2001's Zoolander. With Thunder (opening Aug. 15), he takes aim at the sweetest target of all: actors. Downey plays one of a team of self-indulgent stars cast in the modern equivalent of Apocalypse Now. Stiller plays an action hero who has just adopted a baby from Asia but worries that ''all the good ones are gone.'' Black portrays a comedian known for performing multiple roles in a single film — his latest is called The Fatties: Fart 2. But when the film's director (Steve Coogan) and writer (Nick Nolte) get fed up with their prima donna cast, they drop them into the jungle to fend for themselves. The actors think they're doing some sort of full-immersion filmmaking, but the danger they're in is very real.

Stiller got the idea for Thunder more than 20 years ago while shooting a small part in Steven Spielberg's WWII drama Empire of the Sun. He's continued to develop the script as his own star has risen, which makes taking on his brethren all the richer — watch for cameos from Tom Cruise and Tobey Maguire — and all the more perilous. For starters, Hollywood satires have a rocky box office record. And then there's that little issue of a white guy playing a black guy. Stiller says that he and Downey always stayed focused on the fact that they were skewering insufferable actors, not African-Americans. ''I was trying to push it as far as you can within reality,'' Stiller explains. ''I had no idea how people would respond to it.'' He recently screened a rough cut of the film and it scored high with African-Americans. He was relieved at the reaction. ''It seems people really embrace it,'' he says.

Paramount is hoping so: The studio plans to debut the trailer online March 17, and Downey is all over it. (In one scene, he tries to bond with a real African-American castmate by quoting the theme song from The Jeffersons.) Downey, meanwhile, is confident he never crossed the line. ''At the end of the day, it's always about how well you commit to the character,'' he says. ''I dove in with both feet. If I didn't feel it was morally sound, or that it would be easily misinterpreted that I'm just C. Thomas Howell in [Soul Man], I would've stayed home.''

Are you serious? Everyone knows you don't ever go fully retarded!

Check it out. Dustin Hoffman, Rain Man. Looked Retarded. Acted Retarded. NOT retarded. He could count toothpicks, cheat at cards. Autistic. Sure. NOT retarded. HANKS' Forest Gump. Slow? Yes. Retarded? Maybe. Had braces on his legs, he charmed the pants off Nixon, and won a ping-pong competition. Peter Sellers, "Being There", infantile? Yes. Retarded? No.

You went full retarded man. You never go full retard. Don't believe me? Ask Sean Penn, 2001, I am Sam. Went full retard. Went home empty handed.