"Oh, God. Are you kidding me?"

Unfortunately for Dr. Addison Montgomery, the answer to that question is no. Inside the slick offices of Oceanside Wellness — the setting for ABC's highly anticipated Grey's Anatomy spin-off, Private Practice (debuting at 9 p.m., September 26) — Addison (Kate Walsh) is shocked that her pregnant patient has requested a holistic birth from sexy alternative-medicine practitioner Dr. Pete Wilder (played by the sexy Tim Daly).

She nervously pivots in her scrubs.

"People have babies in hospitals," says Addison. "That's normal! This is..."

"A change of pace?" asks Pete.

You could say that.

When Grey's Anatomy creator Shonda Rhimes first conceived Private Practice, a drama-comedy about a consortium of sexy, not-getting-younger medical careerists in Santa Monica, she had one hope for her second baby: that it not look identical to her first.

"I feel it's lighter than Grey's Anatomy," Rhimes tells Entertainment Weekly, which features Walsh and Private Practice on the cover of its fall TV preview. "It's got all the drama without the surgery! Grey's has always been high school kids with scalpels. These people are a little bit more grown-up."

Funny, Walsh isn't feeling like the picture of maturity right now.

Outside a hideaway café in Los Feliz, Calif. — not far from the house she shares with her new husband, Alex Young, a 20th Century Fox exec — she's arriving 30 minutes late for a meeting.

Rushing through the door, the Private Practice star apologizes repeatedly and sheepishly admits she forgot her wallet.

The 39-year-old actress mutters something about a bad impression, but that's hardly the case, given the commotion she's caused.

Dressed in a simple sundress, face free of makeup, Kate Walsh is an absolute stunner — a throwback to '40s Hollywood, with her Rita Hayworth hair and Katharine Hepburn charm. Customers strain to watch her every move.

Oblivious to her surroundings, Walsh settles at a table to reflect.

"I definitely like writing down goals, where I want to be, how I imagine it. But I didn't write one this down. I didn't write down I'd like my own spin-off," she said.

No one did.

When Shonda Rhimes cast the actress in the first season of Grey's, the plan was to have her around for a handful of episodes, as the no-good tramp who cheated on McDreamy (Patrick Dempsey).

Even Kate Walsh herself planned for a hasty departure.

"After the initial five episodes, I shot another pilot for ABC," said Kate Walsh, who, until Grey's Anatomy and Private Practice 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."

Then a crazy thing happened - 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, Addison became somebody we all could really identify 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 Anatomy 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.

Continue reading the cover story on Kate Walsh in Entertainment Weekly ...

Steve Marsi is the Managing Editor of TV Fanatic. Follow him on Google+ or email him here.