Amy Brenneman, Kate Walsh, Adelaide Kane, Reign

Getty Images, CBS

Forget How I Met Your Mother, we're more interested in how Reign cast Mary's (Adelaide Kane) mother, Marie de Guise, the acting regent of Scotland.

Private Practice star Amy Brenneman landed the part that has history buffs nerding out on the CW royal drama and is set to make her highly anticipated debut in the breakout hit's March 6 outing. And after Brenneman was cast in the pivotal role, executive producer Laurie McCarthy gushed, "We've been waiting for just the right moment, and the actor, to introduce Mary's mother...Amy Brenneman's talent, strength and beauty illuminate even the darker corners of Marie de Guise." And when we chatted with McCarthy, she said her first thought when they started talking about the Marie de Guise storyline was, "When is Amy going to be available?"

Seems like Brenneman was their first and only choice, right? (Especially when you consider the fact that her husband Brad Silberling is an executive producer on the series!) Not so much, at least according to Brenneman, who spills to E! News that she wasn't the first actor in mind for the role—her Private Practice co-star and friend Kate Walsh was!

"Kate Walsh was going to do it. It wasn't really on my radar much, and then Kate, Brad and I all went out, like maybe the end of the summer, beginning of the fall, and she was like, ‘I love the show.,'" Brenneman explains. "They didn't get very far with it because she's involved with some projects that they wouldn't let her out essentially. So I called her up, I was like, ‘Oh my gosh, I'm sloppy seconds!'"

There's no bad blood between the BFFs though as Walsh recently retweeted one of Silberling's tweets about Brenneman's Reign debut, saying, "Go Amy!"

Given their similar features and gorgeous dark locks, you'd think Brenneman would be an obvious choice to play Kane's mother, but Brenneman, who will star in Damon Lindelof's highly anticipated HBO series The Leftovers, tells us she only recently could see a resemblance between herself and her on-screen daughter.

"It's so funny, I was just at a looping today and I had seen the episode, but then I was doing these moments and Adelaide and I really do look alike," she explains. "It's similar colorings and you really do buy it as that dark-haired mother-daughter thing."

Reign airs Thursdays at 9 p.m. on the CW, with Brenneman making her debut in the episode airing on Thursday, March 6.

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