Robert Downey Jr. isn't quite sure how he became an action hero. "Iron Man" came out, did almost $600 million in worldwide box-office business and suddenly his Tony Stark and the red-and-gold superhero suit he built became the stuff of fanboy fever dreams. Sure, there's a [article id="1619391"]sequel coming out next year[/article], but the action star that the Oscar-winning actor feels is the truest fit for his talents is a British investigator whose powers of deductive reasoning are almost as vigorous as his roundhouse kick.

Meet Sherlock Holmes like you've never seen him before in Guy Ritchie's big-budget adaptation, in theaters December 25. "Playing Tony Stark, this super genius, people say, 'You're so cool,' " Downey told MTV News. "He is cool, and sometimes if I get pumped-up enough and take enough creatine, then I can look cool for up to 12 seconds at a time. With Sherlock, it was very different. He's a much more centered and focused person, which works for me. I'm a character actor."

Holmes is also a martial-arts expert and, as Downey put it, "a total badass. He thinks through all eventualities, possibilities and counterattacks when he's fighting, so he actually has the fight worked out before he starts."

One part polished know-it-all, one part bare-knuckle brawler and one part lover -- just not exactly with whom you might think. Sure, Holmes has a love interest -- Rachel McAdams as a former flame who shows up needing his help -- but as even McAdams admitted, the true romance is the one between Holmes and his trusty sidekick, Watson (Jude Law) as they attempt to solve a deadly mystery.

"[I'm] just a woman coming between them," she laughed.

A lot has been made of the somewhat [article id="1617599"]homoerotic undertones of their relationship[/article] -- could these two, gasp, be gay?! -- but Downey remains enlightened and unruffled. "I think bromance is passé," he said with a smile. "We are two men who happen to be roommates who wrestle a lot!"

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"You could say this is a buddy film," Law explained to MTV News, continuing, "whether [Holmes and Watson] will or won't stay together and whether they will or won't drive each other 'round the bend before the end of the case is very much front and center in the story."

In other words, they're like an old married couple. Who fight crime. And kick butt. What's the big deal?

"As a heterosexual couple that at moments could seem gay, they play it off very well," Ritchie told MTV News. "These guys are sort of in love with each other. It's real mateship."

And Downey argues that it stays far closer to Sir Arthur Conan Doyle's 19th-century source material than past cinematic depictions have. And forget about Tony Stark.

"Sherlock Holmes," said the actor, "is easily the toughest job I've ever had."

From the sexy Megan Fox in "Jennifer's Body" to the black-clad warriors of "Ninja Assassin," the MTV Movies team is delving into the hottest flicks of Fall 2009. Check back daily for exclusive clips, photos and interviews with the films' biggest stars.

Check out everything we've got on "Sherlock Holmes."

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