Although “Iron Man” is Marvel Studios’ first self-financed movie  Paramount is distributing  Marvel did not consider casting Mr. Downey to be a risk. “That an actor of his caliber and talent was willing to submit to a screen test spoke volumes about his enthusiasm,” said Kevin Feige, president for production at Marvel Studios. “And his past was not a huge issue. The fact that Disney had already cast him in ‘The Shaggy Dog’ suggested that he was more than ready to do another family-oriented film.”

Iron Man is a thoroughly mortal superhero, the product of Yankee ingenuity rather than a genetic mutation or spider bite. In the film Tony Stark is imprisoned by malevolent jihadi forces in Afghanistan, but uses cunning, heavy metal and an injured but increasingly palpable heart to perform a spectacular jail break. In order to do so, he builds a kind of supercharged exoskeleton that is the other star of the show, an anthropomorphized apparatus that takes some fashion tips from the Transformers but is radical and shiny enough to impress and perhaps excite a jaded action-adventure audience.

A trace of that armor still seemed to be in place early on a recent afternoon in Los Angeles. Mr. Downey used grand flights of rhetoric to glide past questions about his past, dwelling instead on getting mobbed at Comic-Con in San Diego by all the “Iron Man” nerds and rubbing his hands together at the planned global tour on behalf of the behemoth.

When serious actors take on jobs involving comic books and hours in machines and makeup, they generally plug their noses and take the paycheck. Mr. Downey is having none of that. At 43 he is thrilled to be fit enough  he had spent the morning with the living room furniture pushed aside for instruction in wing chun, a Chinese martial art built on aggressive, close combat  to play a hero. He views the Big Comic Book Movie as a kind of arrival after years of lead roles in movies like “The Singing Detective” and “The Gingerbread Man,” which had cinematic pedigrees but little in the way of audiences.

“I’ve been in big movies before and never had a problem with them,” he said, munching a carry-out lunch of sole underneath a gigantic Tobias Keene painting. (one of two in the room). “What is creepy and obvious is that the market was suddenly flooded with morons who thought, ‘If I’ve got $500,000, I can make a baseball cap that has a company name on it and say I’m a filmmaker.’ ”

“On the contrary,” he added, “I am thrilled to have made this movie with Jon. I seem to have been the person who’s had to wait the longest for this kind of gratification.” He leaned forward so that the multi-hued stone medallions on a leather strap dangled as he spoke. “It took a while. Richard Attenborough,” he said, invoking the name of the director of “Chaplin,” “told me that one day your ambition will supersede all of these other impulses you have, and that will help set you straight.”