When rumors surfaced that Tom Cruise would play billionaire industrialist Tony Stark in Iron Man, Marvel Entertainment's (NYSE:MVL) first picture as an independent studio, I got nervous. It seemed like a suicidal choice -- the business equivalent of Stark's alcoholism.

My problem wasn't so much with Cruise the actor as it was the Cruise the movie star. As a longtime reader of the comic, I longed not for someone who'd do well with the inevitable stunts, as Cruise would, but instead for an actor perfectly suited to play the troubled genius that Stark is.

Enter Robert Downey Jr., who, as of yesterday, landed the role. Plagued by many of his own high-profile troubles, Downey remains one of my favorite actors, and a perfect choice to play Stark. Seriously, who better to play a troubled genius than ... a troubled genius, as Downey surely is?

Not that I'm expecting Iron Man to be a deep character study, as Ang Lee's The Hulktried to be. Instead, I'm hoping for an action flick that also dabbles in the dark side of heroism. Iron Man provides that opportunity because Stark, as written in the comics, is a prisoner of war whose life is threatened by an injured heart. He invents an armored battle suit to save his life first; heroism is a distant second.

I'll understand if you think I'm painting a picture of a movie that could be both dark and disturbing. In fact, I expect it to be neither. Instead, Iron Man should be a solid popcorn flick that allows for moments of uncomfortable conflict caused by the psyche of a man who's been through ... well, you know.

Downey is the perfect actor to play that role, which is good news for shareholders. Marvel doesn't need Iron Man to duplicate the success of a certain friendly neighborhood wall-crawler, but it must at least get within spitting distance of Fantastic Four, which booked $154.7 million in domestic box office receipts, according to Box Office Mojo.

Why? According to estimates provided at Marvel's analyst day last month, a film that earns $100 million at the U.S. box office would result in a small operating loss. Losses could quickly pile up in Marvel's credit facility, which compounds interest regularly. A steadily increasing balance would raise the bar for future films to deliver enough profit to pay off the debt and deliver cash to Marvel and its shareholders.

But I find that possibility remote as of today. If handed a good script and smart guidance from film director Jon Favreau, Downey should deliver at least as well as Fantastic Four did, and maybe even a lot better. Go get 'em, Shellhead.

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Fool contributor Tim Beyers says Iron Man is one of his favorite Marvel characters. He hopes ol' Shellhead comes off well on celluloid. Tim didn't own shares in any of the companies mentioned in this story at the time of publication. You can find out which stocks he owns by checking Tim's Fool profile . The Motley Fool's disclosure policy is also ironclad.