Rebecca Mader

It's finally time for Evil vs. Wicked!

Now that the beleaguered residents of Storybrooke have discovered the Wicked Witch's identity, Sunday's episode of Once Upon a Time (8/7c, ABC) will find Zelena (Rebecca Mader) bringing the fight straight into town. Very clearly making her motivations known, Zelena challenges her little sister Regina (Lana Parrilla) to a fight at dusk.

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"It's pretty epic," Mader tells TVGuide.com. "It's definitely the showdown of all showdowns. I couldn't believe it. They basically shot it like a cowboy western. The promo is so fitting. It was a really big night and a big shoot. It's all built to this moment of who's going to win: Wicked vs. Evil. It's a pretty exciting moment in Sunday night's episode."

With Zelena's identity out in the open, you would think she'd now have to change tactics. However, with the Dark One (Robert Carlyle) on her side, the Wicked Witch of the West fears nothing. "She's very set on her end game," Mader says. "She feels very in control of her mission. If you control the dagger, you have nothing to fear."

Well, there is one thing to fear. After a photo of Zelena kissing Rumplestiltskin was released, the fans went crazy online. "That was just in between takes, just hanging out," Mader jokes. "I didn't know that photo was coming out. I saw everyone screaming at me on Twitter. Any fans of Belle [Emilie de Ravin] and Rumple are like, 'What are you doing?!' I'm just not responding because there's nothing I can say other than you'll have to tune in to [the April 20 episode] for that one."

Fans may be even more upset when they soon discover why Zelena is so intrigued by Snow's (Ginnifer Goodwin) baby. The increasing interest has turned the Wicked Witch into something of an enigma. How is Snow's baby connected to Zelena's hate for her sister? A trip to Oz may provide answers. "You're going to get Zelena's backstory," Mader says of Sunday's episode, which features a trip to the Emerald City. "You're going to see when she wasn't wicked and when she wasn't green. It's more of a case of feeling wronged: Why she's so pissed off and why she's become so wicked and what she really wants and how she wants it to go down. There's going to be a lot of stuff revealed, including a lot of different aspects of her personality and her heart."

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This Oz, however, might not be what fans of L. Frank Baum's original novel imagined. "It's got nods to the original, but the writers have put their own wicked flavor to it," Mader says. "It's not a carbon copy. It's a new Once Upon a Time version, which is fun."

Regardless of who comes out on top on Sunday, Mader just feels extremely lucky to have been cast in the role — even if it means hours of makeup to get that shiny green look. "We've got it down now to about an hour, which isn't too painful," she says. "It's pretty intense to get off. We use an oil-based makeup remover, and usually have two people and me scrubbing off until like 4 or 5 o'clock in the morning when you wrap, and you're like, 'Get it off!' Then sometimes I'll get back and the next day I've got like green down the back of my neck. It gets everywhere. I've got it in my hands and my fingernails. Sometimes my fingers are still green when I'm lying in bed at night."

Well, as this Sunday's episode title fittingly puts it, "It's Not Easy Being Green."

Once Upon a Time airs Sundays at 8/7c on ABC.