Downey Jr. plays overly committed actor Kirk Lazarus, a white man cast to play a black soldier in a satire of the performing profession. The film also stars Jack Black and Ben Stiller, who co-wrote, directed and produced it.
"If it's done right, it could be the type of role you called Peter Sellers to do 35 years ago," Downey told Entertainment Weekly magazine. "If you don't do it right, we're going to hell."
Stiller said he was "trying to push it as far as you can within reality," with the intent of satirizing over-the-top actors, not African-Americans.
Downey explained that he kept the character from becoming a caricature because he "dove in with both feet."
"If I didn't feel it was morally sound," he said, "or that it would be easily misinterpreted that I'm just C. Thomas Howell in ("Soul Man"), I would've stayed home."
Paramount is set to release "Tropic Thunder" Aug. 15.