August 13, 2010 at 04:00 AM EDT

When your last film, The Hangover, raked in nearly $470 million worldwide to become the highest-grossing R-rated comedy of all time, you can do pretty much anything you want for an encore. What director Todd Phillips wanted to do was take that film’s breakout star, Zach Galifianakis, match him up with Robert Downey Jr., and set the two loose on a wild cross-country road trip. ”I just knew Zach and Downey would be a fun pairing,” Phillips says.

Downey plays Peter, a tightly wound businessman forced by bizarre circumstances to drive from Atlanta to L.A. with an eccentric and hugely irritating wannabe actor named Ethan (Galifianakis). What appears at first to be a simple odd-couple road comedy develops deeper strains as Peter races to make it home for the birth of his first child and Ethan grieves for his dead father. ”It could have been so two- dimensional,” says Downey. ”It’s not that oh-my-God-we’re-doing-a-dramedy horses—. But I think it’s a step forward for Todd as a filmmaker as far as the emotional depth and complexity.”

Striking the right balance between outrageous comedy and emotional authenticity was a major challenge, but Downey says he was stunned by how easily Galifianakis could shift between the gonzo moments and the more grounded ones: ”Zach acts like he’s just rolling off a log when he’s on the set, but he’s like Neo in The Matrix — he’s thinking faster than everyone else. He killed it. I’ve never been so happy watching somebody do so well in my whole career.”

Whether the R-rated Due Date can replicate the massive success of The Hangover, though, is anybody’s guess at this point. ”Nobody saw The Hangover coming,” Galifianakis notes. ”But now that it has, I can afford to get my Subaru detailed.”

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