Mary McDonnell Mary McDonnell

Major Crimes star Mary McDonnell still isn't exactly sure how she ended up as the star of TNT's newest cop drama.

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The Battlestar Galactica alum first stepped into Capt. Sharon Raydor's shoes during The Closer's fifth season, when her Internal Affairs investigation put her at odds with Brenda Leigh Johnson (Kyra Sedgwick) and her team. Three years later, Brenda is gone and Raydor is now leading the squad on the spin-off Major Crimes.

"I've never had something grow this way," McDonnell tells TVGuide.com. "This was just a delightful three-episode romp, and I was very excited to be able to fully embrace the antagonist. Then it just grew and grew. And I was standing by going, 'Well, this is interesting. Let's take another step.' I don't think it was really until we [premiered] a few of weeks ago that it truly occurred to me what had happened."

And what has happened is a bit unusual. Raydor, once the thorn in everybody's side, is now the series' protagonist. "It was tremendously challenging," she says. "We figured out how to expose all the aspects of her without basically changing who she is — without pretending she's someone other than who we've already known. ... We knew that we couldn't make a false move... because it was too much to ask of the audience. They absolutely loved the problems that Capt. Raydor presented, so we didn't want to rob them completely."

TNT says goodbye to The Closer, hello to Major Crimes

See photos of the Major Crimes cast