By EW Staff
August 18, 2007 at 04:00 AM EDT
Craig Sjodin

When Shonda Rhimes cast Kate Walsh in the first season of Grey’s Anatomy, the plan was to have her around for a handful of episodes, as the no-good tramp who cheated on McDreamy (Patrick Dempsey). Even Walsh had planned for a hasty departure. ”After the initial five episodes, I shot another pilot for ABC,” says Walsh, who, until Grey’s, was best known for playing a fat-suited suitor of Drew Carey on his eponymous sitcom. ”There were always a lot of half-hour shows to do, so I thought, That’s probably going to be my life.”

And then a crazy thing happened in the Seattle Grace elevator: Addison changed from the adulterous bitch who broke Derek’s heart to the winsome beauty who stole ours. ”The more I spent time with her and the audience got to know her, she became somebody we all really identified with,” explains Rhimes, who singularly credits Walsh for making Addison one of the most popular Grey’s characters. ”She looks amazing. She’s smart. She has great comedic timing, but can also get the big serious moments. Plus, she’s great to work with.”

A handful of episodes turned to 53, and by September 2006, when Rhimes was editing the Grey‘s two-parter from January 2007 in which Addison tells McSteamy (Eric Dane) ”I did want a baby, I did. I just didn’t want one with you,” she began envisioning a spin-off for her accidental star. ”I was trying to figure out a way to complete the character for season 3, and give her the journey that had been waiting for her,” says Rhimes.

(To read the full cover story about Kate Walsh and how Private Practice came together, click here.)