With Frozen on the horizon in the Once Upon a Time universe, what else can viewers expect from the new season?

At the end of last season, Emma and Captain Hook inadvertently left a portal open, allowing just enough time for Frozen's Elsa to slip into town. Plus, there was the awkward love triangle between Regina, Robin Hood and his once-dead wife Marian, Mr. Gold and Belle's marriage lie and the Charmings' venture into parenthood. As Jennifer Morrison tells The Hollywood Reporter, the premiere contains "a mess of information" that she promises will "come together."

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Ahead of Sunday's return, the Once stars talk to THR about season-four's biggest non-Frozen questions.

Can Captain Hook and Emma be in a healthy relationship?

Things were looking good for the happy couple at the end of season three but in Storybrooke, that bliss is only temporary. "They've really invested in maybe wanting to be together," Colin O'Donoghue tells THR. "Emma is The Savior and Hook is a 200-year-old pirate — it's never going to be plain sailing or easy going. But they're going to try their best to make their relationship work." In doing so, Hook — who will "do whatever it takes, good or bad" to maintain their romance — will be forced to decide whether "he can find a place for Emma within his heart when she's so busy trying to save the world," O'Donoghue adds.

Morrison echoes that sentiment, saying that though Emma and Hook both desire "to be the better versions of themselves and vulnerable enough to be close to someone romantically," it's easier said than done, especially when more immediate threats like Elsa's snow monster loom. "There's a lot of trying to work out a relationship in the midst of a lot of chaos, which is funny at times and heartbreaking at times, and causes them to really figure out how this could or couldn't work," she says.

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Who will Robin Hood choose: Marian or Regina?

After the unexpected return of Marian, Robin Hood is now faced with a major dilemma, one that is addressed head-on in the show's return. Does he honor his vow to Marian or does he start anew with Regina? A decision is made. "I have a certain living situation I have to deal with, with a current romance and a wife coming back from the dead," Sean Maguire tells THR. "That seems to be the situation last season ended with. Suffice it to say, Robin Hood does come to a truce of sorts."

Regina is understandably frustrated by the sudden turn of events, which may veer her off her path of goodness. "We'll see Regina really battling the two sides of good and evil. She's been on the road to redemption for the last couple of years and yet she's been evil for most of her life," Lana Parrilla tells THR. "Where is she really comfortable living? And is she going to continue down her old habitual ways or is she going to surprise us all and do the right thing?"

How does Emma cope with the guilt of causing Regina's unhappiness?

Well, it's complicated. "Emma can't apologize for saving a life, but it's not her plan to mess up Regina's true love," Morrison says. "She's trying to toe the line. She's trying to prove that she truly is Regina's friend and that she truly wants what's best for her, but also protect Marian." Juggling a vengeful Regina won't be an easy task for Emma and that becomes an ongoing season-four thread. "Ultimately, Emma is trying to keep Regina in a good place in her life. She's worked so hard and come so far from her villainous mindset that it would be tragic, in Emma's eyes [if she returned to that] because she's become Emma's friend," Morrison says. "[Emma]'s going to spend time fighting to prove her friendship and fighting to keep Regina in a good place."

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How do David and Mary Margaret navigate parenthood?

"It's hard because they're finding this all out as they go. It's basically the first time they've [raised a child]. They don't want to lose [baby Neal] like they lost Emma," Josh Dallas tells THR. "Emma has suffered a lot of psychological repercussions because of it. She's isolated herself because of it. They want to raise Neal right and still honor Emma as their child." Says Ginnifer Goodwin, Snow and Charming "are confronting a lot of their exceptionally guilty feelings in having lost the opportunity with Emma, who has a remarkable story with Hook."

What about Charming's past?

Attention will be given in the first half of the season to Prince Charming's "origins," Dallas hints. "We're going to see, in my opinion, the most important friendship he's ever had outside of Snow White that really changes the entire course of his life and sets him on the path of becoming the hero that he is. It's an unlikely connection with a character that I can't wait for people to see."

Will Mary Margaret get back into the workforce?

The simple answer: Yes. Though her immediate focus will be to raise Neal, Goodwin says, she will find herself taking on a new job. "She has been thrust in a brand-new job while she's trying to juggle [raising a newborn] and she's expected to rule as she is technically the ruler of the Enchanted Forest transported to Storybrooke," she says. Morrison hints that Mary Margaret will be taking over a position of considerable power for someone else within Storybrooke.

What is Mr. Gold and Henry's dynamic this season?

Henry will find himself under Mr. Gold's guise, which may not be a good thing. (He'll also go on a quest with Regina; Parrilla teases, "You'll see them working together in ways they haven't before.") "It's going to be a lot of fun," Jared Gilmore tells THR of Henry and Mr. Gold's story. "Henry is going to learn a lot this season, which is going to be great for him because he's going to learn what he's meant for."

How much trouble is Mr. Gold in?

"Belle hasn't heard any of [Mr. Gold's] secrets at all," Robert Carlyle says, referring to beginning his marriage to Belle with a lie. Carlyle hints that though things may seem perfect between the newlyweds at the start of the season, "it's going to be bad at the end of episodes nine and 10 for Rumple."

What has Will Scarlet/the Knave been up to?

There is some mystery surrounding Michael Socha's Once debut (he doesn't appear in the premiere) and co-creator Eddy Kitsis said that the context in which Will Scarlet re-emerges post-Once Upon a Time in Wonderland will be provided — in due time. "We are going to tell that story, what happened and what brought him back to Storybrooke," he said, hinting that Will Scarlet "has an interesting connection to another character from... season one." Ask Socha his theories on what Will Scarlet has been up to and he's just as stumped as you are, "I sit there all night thinking of all sorts of different [theories] but none that I'm solid with."

One thing is for certain, the Knave has unfinished business with Robin Hood. "There is some stuff between us that needs to be resolved," Maguire says. "We delve into that at some point in the season. You know he's there and you know I'm there, so it's inevitable we will talk at some point."

How does Hook adapt to modern clothes?

Over the course of the season, the pirate will shed his leather coat for more contemporary clothing — a change O'Donoghue was particularly excited about. "It was nice because I've been wearing a 50-pound leather jacket [for two seasons]. It gets pretty hot," he says. Hook's new look, which O'Donoghue likens to "a '70s rockstar," still maintains "an element of who he is, even though he's trying to fit into this world. It's his idea of what it would be like."

How many Disney nuggets are in the premiere?

At least half a dozen — and one Disney reference may offer a hint to a story line some of the core Once characters will be embarking on. (Another moment, the classic Beauty and the Beast ballroom scene, was revealed in promos.) "That was very intentional," Morrison says of the various gems. "It was very important to have those DIsney nugget moments. If we do gain audiences because of adding Frozen, [the producers] wanted to give them touchstones of what the show's about."

Once Upon a Time returns at 8 p.m. Sunday on ABC.

Email: Philiana.Ng@THR.com

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