Channing Tatum Back at Work in 'Hardest Acting Challenge' Yet

By Tim Nudd

12/13/2012 at 10:45 AM EST

You can't just hang around being sexy all day. At some point, you have to get back to work.

So it is for Channing Tatum, PEOPLE's Sexiest Man Alive, who was spotted Wednesday with a barely recognizable Mark Ruffalo on the Pittsburgh set of the movie Foxcatcher.

Tatum, 32, and Ruffalo, 45, play brothers Mark and David Schultz in the film, which is based on the real-life story of U.S. Olympic wrestler David's murder at the hands of team sponsor John du Pont of the famed du Pont family.

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The serious role is stretching Tatum's abilities. "It's going to be the hardest acting challenge I've had to date," the actor, who plays Mark Schultz, recently told the Los Angeles Times of the movie.

"It's a bit of a weird science experiment to put these guys up on the screen, because no one really understands what happened or why [the murder] happened."