Grey's Anatomy

It's like the first day of school as Caterina Scorsone hits the set of Grey's Anatomy. She's there to begin filming the big Grey's-Private Practice crossover. Bubbly and bright-eyed, the new girl roams the halls of Seattle Grace with a sense of awe.

"No matter what, when you start a new situation like that, especially with these people that you've loved and admired for so long, you're going to be nerve-wracked," Scorsone tells TVGuide.com.

Her arrival is the moment Grey's fans have been waiting for since Scorsone joined Private Practice as Derek Shepherd's estranged sister Amelia. In Thursday's episode, Amelia will come back into Derek's life, opening up old wounds and shedding new light on McDreamy's past.

Private Practice's Caterina Scorsone heading to Grey's Anatomy

Patrick Dempsey says the pair clicked immediately. "Seven years and suddenly she's here," the actor says, while lounging between scenes in a Grey's-branded golf cart just outside the Seattle Grace set. "We look like we could come from the same mother and father."

("Whew," Scorsone says, relieved to hear he thinks so.)

Amelia comes to Seattle in hopes of reconciling with Derek, who hasn't spoken to his sister since her downward spiral into drugs following their father's murder. "It hasn't been dealt with for many years," Dempsey says.

"Amelia is just a bit of a wild card generally," Scorsone says. "She'll do whatever will give her a bit of a high." Her latest fix: The mile-high club. "You know it's exciting," Scorsone says with a devilish smirk. Amelia meets a stud on the plane and through their, ahem, interaction, deduces that he may have a brain tumor. Since Derek is also a neurosurgeon, the new patient might be the perfect ice-breaker.

Having quickly sussed out his sister's mile-high misdeeds, Derek doesn't jump on the offer. But Meredith (Ellen Pompeo) wants to help mend their relationship and attempts to intervene.

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Pompeo, who between takes playfully reads the crew their horoscopes from a magazine, says, "Meredith sees him resisting and has to get to the bottom of why he's resisting. I look at [the crossover] more as another side of Derek that Meredith is getting to know."

While Amelia was reluctant to go to Seattle, Scorsone still believes her character is genuinely sorry for the behavior that pushed her brother away. "She misses Derek a lot," Scorsone says. "She's as sorry as she can be at this point in her life. For anyone to make a real apology to someone, they have to fully understand every aspect of what it was that they did, and why they did it."

Will there be resolution for the Shepherds? "I don't think they can have a good relationship, not on this show," Dempsey says, adding that the core group of doctors at Seattle Grace formed a family because of their inability to have good relationships.

Scorsone says that to be able to find some common ground, the pair will have to work out their individual problems first. "Both of them experienced a huge trauma together, so they both have to be healthy enough to talk about it with each other," she says.

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The docs will definitely find some commonality when they share an OR. "Shepherd/Shepherd" was spotted on the OR board — or as Scorsone likes to call it, "Shepherd squared."

"[Derek] doesn't trust the Amelia that did all those things way back when," she says. "She definitely wants to impress him and show him that she's not the same little girl that was so disappointing to him."

The crossover is not all heavy sibling drama, though. It wouldn't be an episode of Grey's Anatomy without Dr. Mark Sloan (Eric Dane) flirting with a Shepherd sister, right? Scorsone would only say that Sloan has slept with some of the other sisters. "He just has that energy with pretty much every woman."

The Grey's Anatomy-Private Practice crossover airs Thursday at 9/8c on ABC.