LOS ANGELES (Reuters) - It took comic book heroics and major studio clout but when "Iron Man" debuts in movie theatres this week, Robert Downey Jr. will have completed a roundtrip from showbiz boy wonder to fallen star and back again.

Downey, 43, is happy about it, not so much for being back in business because, in recent years, he has been working steadily in many low-budget and independent movies.

Rather, he is happy because he is back in a starring role in a big Hollywood flick — the first major release of the summer season — with the kind of marketing muscle that means "Iron Man" will be seen by millions of fans.

It is the kind of attention he has not enjoyed in years.

"You want to reach an audience and you want some surety that all your work did not all go for naught," Downey told Reuters. "'Iron Man' has this big, gigantic PR (public relations) machine. It's been a wild ride and I feel we made a quality product."

Downey was born to act. He is the son of New York filmmaker Robert Downey Sr. who began his career in his father's films and grew up to claim a spot on TV's "Saturday Night Live."

By 1987, he had burst onto the Hollywood scene as a highly touted young actor in "Less than Zero." In 1992, at age 27, he was nominated for a best actor Oscar for his portrayal of screen legend Charlie Chaplin in "Chaplin."

Then, Downey's life spiraled downward from alcohol and drug abuse, and in 1997 he was jailed for violating probation on drunk driving, drug possession and a weapons charge.

About six years ago, Downey has said he realised it was time for him to get his real-life act together. He found work on TV's "Ally McBeal" and in little-seen movies like "A Guide to Recognizing Your Saints."

Then, along came "Iron Man" director Jon Favreau and Marvel Studios, which owns the comic book character and made the movie.

They were casting for a 40-ish actor to play Tony Stark — the genius chief executive officer of a weapons maker who, after being captured in Afghanistan, decides to make a super high-tech suit of armor to defend the defenseless.

"He brings a subversive sensibility to this very mainstream film. It adds other dimensions to it — humor and humanity," Favreau said, likening Downey's offbeat casting to Johnny Depp in the hugely popular "Pirates of the Caribbean" films.

Downey likens the role of Tony Stark to that of Jason Bourne (Matt Damon) in the "Bourne Ultimatum" and "Bourne Supremacy" movies that pair big Hollywood action with a flawed central character who is complicated and struggling to find himself and his place in the world.

"This is someone who is literally and figuratively almost destroyed by their own (mistakes)," Downey said.

In the movie, Stark (aka Iron Man) eventually sees hope and redemption when he looks in the mirror and into the eyes of his devoted assistant, Pepper Potts (Gwyneth Paltrow). And "Iron Man" is not with out its action — there are plenty of battles.

Yet "Iron Man" is winning good early reviews not so much for the action but mainly because of Downey's performance.

Downey, who moved into an office next to Favreau to work closely on the film, said he just wants the film to be seen.

"I just want to make good movies that people are going to see," he said.