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"Grey's Anatomy's" Kate Walsh and Patrick Dempsey

"It's a great fantasy and device for the writers to turn characters on their head and for fans to be amused," guest star Kate Walsh says of Thursday's big "alternate universe" episode of Grey's Anatomy. In the hour, Meredith ponders what life could have been like had she grown up with a normal mother and made a series of different choices.

"As things start to unfold, we in the writers' room kept coming up with more and more things if Meredith had been a shiny, happy person," Grey's creator Shonda Rhimes tells The Hollywood Reporter, noting that Derek would have remained unhappily married to Addison (Kate Walsh).

Here's a character-by-character breakdown of who's up to what in the bizarro world.

Ellis (Kate Burton): She's happy, Alzheimer's free and chief at Seattle Grace and married -- to Richard! -- and runs a very tight ship while still being in control of Meredith, who still lives at home. "She's pushing her daughter to be better, to be more," Sara Ramirez tells THR. Talk about too much family time.

Richard (James Pickens Jr.): He's the nice guy to Ellis' fierce hand at the helm of the hospital, often the quiet confidant to others at the hospital, including Bailey. He also seems quiet content to be the family man and let Ellis take center stage at the hospital, despite not always agreeing with her choices.

STORY: 'Grey's Anatomy': Watch the First 10 Minutes of the Alt Universe Episode, 'If/Then' (Video)

Meredith (Ellen Pompeo): She's happy, peppy, perky and clad in pink. Plus engaged to Alex, who after some initial grousing has Ellis' blessing. Oh, and she's no longer Dr. Grey, instead going by Dr. Webber. "To me a happy Meredith, a chirpy girl who wears pink is a horror story to me. That girl is sort of horrifying," Rhimes says.

Derek (Patrick Dempsey): Kate Walsh tells THR that Derek "isn't at his best" in this world and isn't the top doc at Seattle Grace, with Ellis often picking on him. "I call him McDreary but he's not the happiest on the planet," the Private Practice star says with a laugh. And there's clearly no Mer-Der. Says Rhimes: "[In this world, Meredith] never would have been a girl in a bar, meeting a boy in a bar and that never would have happened. So if she'd never been in the bar, she never would have ended up with him. Which means when Addison showed up to get her husband back, she would have just gotten him back."

Addison (Kate Walsh): Walsh says Addison is pregnant with her and husband Derek's first child and that on the surface they seem to be the hospital's power couple. "Addison is pretty established in her career at this point and she's seven months pregnant and probably thinking more about family and her personal life more at this point," she says. In addition, she's very tight with Ellis and is sort of like a "class pet" for her. "It's great thing for fans who have fantasized about Addison and Derek staying together and what would have happened. It's a great fantasy and device for the writers to turn characters on their head and for fans to be amused."

STORY: 'Grey's Anatomy's' Shonda Rhimes and Sara Ramirez Ponder Destiny in 'If/Then'

Alex (Justin Chambers): Rhimes says Alex is "both horrifying and hilarious" in the alternate universe -- have you seen those glasses and that happy-go-lucky attitude? Understatement! Despite slipping a ring on Meredith's finger, the couple keeps their relationship private at the hospital even though Ellis has signed off on the union.

Callie (Sara Ramirez): Married. Three kids. Owen. And have you seen her kiss him? Ramirez says there are no matter what page you turn to, "at some point in your life, your destiny catches up with you and finds you no matter what." Without having ever met Erica Hahn, Rhimes says Callie never discovered she might have feelings for a woman. "She's pining for a woman and doesn't know why and she's sort of wistful but she's trapped," the showrunner says. "It's this really lovely moment. The part of the episode that I think is really magical is, can they all find their way back to who they should really be. That's sort of what you're watching. I think it's really delightful." Beyond her marriage and motherhood, Ramirez says Ellis has taken a particular interest the cardio surgeon and serves as a mentor to her.

Arizona (Jessica Capshaw): Capshaw tells THR that Arizona's sexuality "is not discussed" in the hour and that her relationship with Callie starts off on the wrong foot and progresses from there. "There is a growth -- there's the beginning where they lock heads over a medical situation and then there's the growth to an understanding and an affinity," she says. "But I don't think it's clear that there's something between them that's outside of that." Beyond Callie, Capshaw says viewers will see a different, "sexier, more sophisticated" Arizona in this world, who doesn't have a partner and is really just on her own.

STORY: 'Grey's Anatomy': Who's Kissing Whom in the Alt Universe? (Exclusive Video)

Owen (Kevin McKidd): He finally has the family that he's always wanted -- Ramirez says Owen is a "great dad" -- but he's still not happy. "[Callie and Owen] have no business being together: he's pining for somebody else, she's having all these unexplained feelings for Arizona," Rhimes says.

Teddy (Kim Raver): With Owen still suffering from PTSD, still Skypes with his "buddies" in Iraq -- including, as Callie believes, a guy named Teddy.

Bailey (Chandra Wilson): "Without Richard to spend the time mentoring her, Mandy Bailey might have stayed Mandy Bailey and never gotten to be the Nazi," Rhimes says. Indeed: She's incredibly timid and allows Ellis to snake a high-profile surgery right out from under her.

Cristina (Sandra Oh): In this world, Cristina is completely ostracized, with Callie never really getting her. "You're going to see Cristina struggle with a few things," Ramirez warns. "She's smart through and through but maybe her personality has been a little bit affected by making different choices."

April (Sarah Drew): Still single and, apparently accepting that her "person" -- Meredith -- is now engaged to her former crush, Alex, April now carries a torch for Charles (Robert Baker) who is still alive since the hospital shooting that claimed his life in the typical Grey's world never happened.

Jackson (Jesse Williams): So far, little has been revealed about Jackson's alternate universe persona, other than that he winds up treating a very different Lexie.

Lexie (Chyler Leigh): Little Grey, it turns out, appears to be the dark and twisty one, having come into Seattle Grace covered in tattoos and piercings as a patient suffering from an overdose of cocaine.            

Mark (Eric Dane): Mark never winds up coming to Seattle Grace, Rhimes says.

Izzie (Katherine Heigl) and George (T.R. Knight): Ramirez says the duo are mentioned briefly, while Rhimes teases that viewers will "understand where they went in this new world." Don't bet on seeing either.

Hit the comments wi
th whose story line are you most excited to see in the alternate universe. Grey's Anatomy airs Thursdays at 9 p.m. on ABC.

Email: Lesley.Goldberg@thr.com; Twitter: @Snoodit

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