Jack Black has signed on to star in the Ben Stiller-helmed comedy "Tropic Thunder," with Robert Downey Jr. and Jay Baruchel.

In the film, everything goes wrong during the making of a big-budget war movie, and the actors end up becoming the commandos they are playing reports Production Weekly.

Black will play Jeff "Fats" Portnoy, an overweight gross-out comedian, who's forced to kick his drug addiction while filming on location in the jungle. Downey will play Kirk Lazarus, the greatest actor of his generation and a four-time Oscar winner. Baruchel will play Kevin Sandusky, an unknown actor on the set and Stiller plays Speedman.

Stiller, Justin Theroux and Etan Cohen penned the screenplay. Shooting is set to begin in July.