Adam Horowitz and Eddy Kitsis talk with THR about the final three episodes of the season and changing the structure of season five — plus watch an exclusive clip from Sunday's episode.

As Once Upon a Time approaches its fourth season finale, key relationships are on the verge of shifting.

Although the Queens of Darkness was the show's sell going into the second half of season four, the series is down to just one — Maleficent (Kristin Bauer van Straten) — and, in theory, she's about to get her happy ending with the return of her daughter, Lily (Agnes Bruckner).

Meanwhile, things are less happy for the former Evil Queen (Lana Parrilla). Regina learned in the last hour that not only had her evil sister, Zelena (Rebecca Mader), been impersonating Marian (Christie Laing) — aka the wife of Regina's true love, Robin Hood (Sean Maguire) — but Zelena is now pregnant.

The Hollywood Reporter caught up with Once Upon a Time executive producers Adam Horowitz and Eddie Kitsis to break down what to expect from the maternal shenanigans, Emma's (Jennifer Morrison) upcoming test, Rumple (Robert Carlyle) and how the show's split-season approach will change for its expected fifth season.

There's a lot of mama drama — Zelena's pregnancy, Lily's impending return to Storybrooke and Regina's mother, Cora (Barbara Hershey), featured in flashbacks. What can you preview about Sunday's hour?

Eddy Kitsis: This Sunday we're going to get into all the mama drama. We are going to get a big, juicy, Robin Hood-Regina, "What the hell do we do now?" scene in a bar in New York City.

How much can you play with pregnancy options on a show that airs in the family hour at 8 p.m. on a Disney-owned channel?

Kitsis: This entire week's episode is literally about those questions, and what do they do?

Adam Horowitz: The episode's titled, "Mother," and it plays with that idea in a number of ways.

Is Zelena a character you view as redeemable at this point?

Kitsis: What we've tried to do on the show is to make no one completely good and no one completely bad. We've had Snow White (Ginnifer Goodwin) do something horrible and Rumple do something beautiful. Where Zelena ends up is truly up to her. Everyone's worried about Emma going dark, but for this weekend, I'd be more worried about Regina.

Given Regina's struggle to remain on the right path, how is she going to handle this curve ball?

Kitsis: That's what the episode is about: what does she do? It's so hard to be good, it's so much easier to tap into your darker temptations. It's so much easier to throw your computer versus patiently try to fix it.

What can you tease about the Cora flashbacks?

Kitsis: Cora is one of our favorite characters to write for. This is one of our flashbacks that's really about getting into the character, and the psyche of Regina; learning something new about her as a character. It's not like, "Oh my God, here's a huge mythological flashback!" It's a flashback between a mother and a daughter, that really sets up Regina and her state of mind in Storybrooke. We realize there was a time in Regina's life where her mom tried to give her advice about who her real enemy is.

How will Lily and Maleficent be handling their reunion?

Kitsis: That it is going to play out as neither of them expected. Lily is going to find herself surprised.

How does this change Maleficent going forward?

Kitsis: Watch Sunday, because we ask those very questions, and they get answered. We're going to literally see how she reacts to her daughter, and what she wants to do going forward.

See more 'Once Upon a Time': Key Disney Characters (So Far)

Emma seemed to resist indulging her darker impulses in the last hour. What will that struggle be like going forward?

Kitsis: Emma is going to be tested like she never has before. There will be some character growth, some emotional growth and truth in how she feels about Hook (Colin O'Donoghue). We're going to see her really fight against that darkness, and she'll really show heroism at the end of the year.

How is that playing into her relationship with her parents?

Kitsis: It plays into everything. You're going to see the messiness this weekend of her and her parents and how she forgives them. I think she's going to be tested these last few hours.

Snow and Charming have had a lot of angst about what they did to Maleficent. How will they be handling the reunion between Lily and Maleficent?

Kitsis: I think they feel a lot of guilt, so for them, reuniting Lily with Maleficent will kind of start to ease their conscience.

Rumple seemed to do the right thing when it came to protecting Belle (Emilie de Ravin), and accepting her relationship with Will (Michael Socha). How will that triangle be playing out in the final hours?

Kitsis: That triangle, we're going to get a resolution into that. We're going to get more insight into why Rumple is behaving the way he has. We're going to see him in a new way.

Will it be something we learn via flashback or something that happens in present day that makes us re-evaluate things?

Kitsis: It will be both. And also in another way.

Horowitz: Not quite flashback, not quite present day.

What can you say about the Author's (Patrick Fischler) journey in the final episodes? Will he be handling his grief over losing Cruella (Victoria Smurfit) in a healthy manner?

Horowitz: The Author is dealing. He's mourning the loss of Cruella, as we saw. But boy, are we going to see him activated in a way we have not yet seen.

Kitsis: He may wonder what it could have been like. But let us not forget, she did want to kill him. In the beginning of the year, Regina said in the premiere, "I want to find the Author, Operation Mongoose, and make him write me a happy ending." Our finale is called "Operation Mongoose, Part 1 and 2" — we're going to make good on what happens when the Author starts writing.

Read more 'Once Upon a Time's' Patrick Fischler on the Author's "Layers," History With Cruella

Will we be seeing the Authors who came before this current one?

Kitsis: You are going to see how the Author got the job.

Will that be the theme of the flashbacks for the finale?

Kitsis: That is a bit of the flashback of the finale. The finale is a bit of a two-hour concept movie.

Horowitz: It's structured a little bit differently than we've ever done before. There will be journeys into other worlds, times and places, but not quite in the way we've done it.

The norm in previous seasons was to intro the next arc in the final episode of the current one. Will that plan still be in effect this year?

Kitsis: We certainly are setting up for what next season would be at the end of the finale this year, but we're doing it in hopefully a different way than we've done before. It's probably a little more reminiscent in a way to season one, where a condition [was set out] that magic is coming, versus having a new villain of the year or a journey to a different world. It'll be in a slightly different a way, but hopefully just as earthshaking or changing than we've ever done, if not more so.

So is that to mean we won't be getting any new characters for this upcoming arc?

Kitsis: We're not introducing any new characters [in the finale], we're just changing the condition of the town.

Interesting. What else can you share about the final episodes?

Kitsis: We're going to see some familiar faces.

Horowitz: We're going to see August (Eion Bailey).

Kitsis: We're going to see some of the people we've seen this year. The dwarves, the Blue Fairy (Keegan Connor Tracy), but in ways we've never seen them before.

Horowitz: It's one of our favorites.

Can't wait for Sunday's hour? Check out an exclusive sneak peek of "Mother" now.