by William Keck April 12, 2010 10:38 AM EST

She’s been practically clawing her eyes out to meet the cast of her favorite show, and now Demi Lovato’s got her chance. When the 17-year-old Sonny With a Chance star arrived on the Grey’s Anatomy set to shoot her May 13 guest appearance, “The first two actors I met were Justin Chambers and Jessica Capshaw, and I was like, ‘There’s Karev and Arizona!’ Then I met Patrick Dempsey.” On a tour of the hospital set, she picked up a patient’s chart to pretend she was making rounds. But Demi’s not playing a new med student; she’ll be a frightened patient named Hayley, who might suffer from schizophrenia (see the claw marks in this exclusive first-look photo).

A new fan to Grey’s, Lovato began catching up on the series during breaks on the set of Camp Rock: The Final Jam, a Disney Channel TV-movie sequel airing in July, which teams her up again with real-life beau Joe Jonas. “Grey’s is a dream role for her,” says Joe, though he prefers Top Chef over medical dramas. “It’s great people are going to see her do different things. I’m really proud of her.”

The episode will also showcase just how good a doctor Karev has become. “Alex has definitely worked on his bedside manner,” says Justin, whose character will continue to get a workout between the sheets with Lexie (Chyler Leigh), now that he’s divorcing Izzie (Katherine Heigl). Having exchanged text messages with Katie since her official exit, Justin bears her no ill will: “Who can blame her for striking while the iron’s hot? We’ll remain friends.”

With Isaiah Washington and T.R. Knight also gone, and Ellen Pompeo saying she’ll be next, Justin’s one of the few remaining originals who’s enthusiastic about signing on for more seasons when his contract comes up for renewal next year. Turning the big 4-0 in July, he plans to spend his hiatus in Los Angeles starring in a movie opposite Chyler, who’s also producing the indie thriller with her husband, Nathan West. “With five kids, I don’t want to be traveling the world from one film to the next,” says Justin. “I want to stay here and watch my kids grow up. Grey’s Anatomy is a good job for my lifestyle. I like it here.” Hear that, Heigl? That’s what gratitude sounds like!

Source: TV Guide Magazine