More established as an actor for his roles in American Psycho, Charlie's Angels: Full Throttle and Six Feet Under, Theroux recently had a hand in the script for Ben Stiller's upcoming war comedy Tropic Thunder.
He is the cousin of BBC documentarian Louis Theroux.
Iron Man, Marvel's first self-funded comic book adaptation, has so far grossed more than $500 million at the worldwide box office.
Director Jon Favreau and star Robert Downey Jr. are both expected to return for Iron Man 2, which has a release date penciled in for April 2010.
