You've heard the adage, "Stick with what you know" -- and in Patrick Dempsey's new movie, he's doing just that. He's playing a doctor.

In the romantic comedy "Valentine's Day," he'll play a married obstetrician who's secretly dating Jennifer Garner's character on the side. Sly devil.

The movie follows five slightly interconnected stories in Los Angeles during Valentine's Day -- and Patrick is joining an all-star ensemble cast that already includes Jennifer, Julia Roberts, Anne Hathaway, Jessica Alba, Jessica Biel, Shirley MacLaine, Bradley Cooper, Ashton Kutcher, and Topher Grace.

His last foray to the big screen was 2008's romcom "Made of Honor." Shooting for "Valentine's Day" starts this week, and Patrick will be done in time to return to "Grey's Anatomy" this fall -- where he hopefully doesn't become another cheating doctor after he marries Meredith. We're watching you, Patrick ...

PHOTO CREDIT: ABC