Winter is coming early on Once Upon a Time.

ABC's fairy-tale drama heads into its fourth season with a chilly 11-episode arc that introduces the beloved characters from Disney's Oscar-winning (and buzzy) animated blockbuster Frozen such as Elsa (Georgina Haig), Anna (Elizabeth Lail) and Kristoff (Scott Michael Foster) into the Once universe. It's a decision co-creators and executive producers Adam Horowitz and Eddy Kitsis hope pays dividends.

"There’s always pressure when you’re making a show to try to outdo yourself, and I think in this case, there’s a huge amount of — not just pressure, but responsibility that we feel," Horowitz tells The Hollywood Reporter. "These are beloved characters who we want to do justice and we’re doing our best and we’re very excited about what we’ve come up with."

Ahead of the Sept. 28 return, Horowitz and Kitsis talked to THR about the pressures of folding in the Frozen universe in an organic way, the aftermath of Elsa's unexpected Storybrooke debut, the future of Emma and Captain Hook's romance and much more.

Can you clarify the timeline for the new season?

Adam Horowitz: Without giving the exact date, which we don't have in front of us, we are roughly in early 2013 given that the show started in October of 2011. We’ve been more or less running in real time, including the year time jump. Because of that, the trailer for Frozen hasn't even premiered so in the world of our show, none of the present-day Storybooke characters are aware of the Frozen mythology and universe.

Has that been a tricky tightrope to walk in terms of the audience’s knowledge versus the characters’ lack thereof?

Eddy Kitsis: Hopefully you're watching for the story. When I watch True Detective, I’m not like, "Wait, is this Thursday?" Hopefully the audience isn’t asking what date it is, because what we do is we pick up from where we ended last year, and the way we introduce the Frozen characters into the show is pretty organic. I think that the timeline isn’t really a question — what people will see is the story unfolding before them.

Horowitz: And yes, it is a slightly different approach in that starting with the pilot, everybody knew who Snow White was and that was part of Emma’s disconnect with being able to believe what was going on in Storybrooke, because she was so well aware. We're instead approaching these characters as though they are a real part of the universe that we’ve created. And while our real, old characters aren’t aware of them, they are, as you will see, connected into the Once universe — and that’s really where our hopeful twist comes. We’re not trying to change these characters or redefine them, because we love the movie so much and what they did with them. We’re instead trying to surprise the audience with how they become involved with our characters and our world.

So the events of the Frozen movie took place in fairy-tale land before 2013, since we see Elsa and Anna in Arendelle of the past preparing for Anna's wedding to Kristoff.

Horowitz: The way to think about it is, divorce yourself from time and space as you know it and think of the world of story occurring in a timeless place. What we’re not saying is that the events of the movie took place in November of 2013. What we’re saying is the movie came out and told the story that occurred in the land of our fairy-tale world. As far as the timeline goes, when we see Elsa arrive at the end of season three, that is an Elsa who is post the events of Frozen. Going forward, the Elsa that we are seeing in Storybrooke is someone who experienced everything you saw in that movie and is hopefully that character who is now facing a new challenge and involved in a new mystery that we intend to explain in the first episode.

The show is known for putting its own twist on iconic fairy-tale characters. Should we expect the same treatment for the Frozen gang?

Kitsis: There are times we take liberties as we’ve done with Peter Pan and other characters, but Frozen we really feel like they are so well-defined and we were so inspired by what we saw up on the screen last November, that we don't want to change those characters. It’s much more about Anna — what we love about her is that she’s fearless and she never gives up on the people she loves. She has no problem going off into the middle of the storm to find her sister. What happens when a true believer like that gets in front of the devil himself, Rumplestiltskin, who sees the worst in everyone? What happens with these two kinds of characters? In fact, what happens when Elsa, who is somebody who wanted to run away, meets another character who loves to run away, Emma? It was really about keeping the characters exactly who they were in the movie but seeing how they would interact with other characters on Once Upon a Time and telling a new story.

Love, whether it be of the romantic kind or family, is a big part of Once's DNA. In the film, Elsa did not have a love interest, unlike Anna. What's the likelihood Elsa will have one here?

Kitsis: She will not. And even though we showed a clip at Comic-Con that showed that Anna was engaged to Kristoff, we can tell you that what we really responded to in Frozen was that it was about these two sisters. Our season one ended with true love’s kiss breaking between a mom and a son, and for us we’re really excited about the idea of the sisters and that story much more than Elsa finding a boyfriend.

How complicated will things get between Anna, Elsa, Kristoff and Prince Hans on Once?

Horowitz: The relationships between all those characters — Anna, Elsa, Kristoff, Hans — we’re picking up where they were left off in the movie, so there is a complexity and conflict there. But at the core of it, the story we're trying to tell with the Frozen characters is a story about sisters, and that’s something we want to bring to our show.

How are you ensuring that the non-Frozen characters continue to get screen time while you mine this new world?

Horowitz: What we’re trying to do is not make the show just the Frozen show. We want to tell this Frozen story, but as you’ll see, their story is going to relate to our core characters from Once Upon a Time.

Kitsis: It was very important that it still [feels like it's] Once Upon a Time. It’s not the Frozen sequel; it is the characters from Frozen coming to Once Upon a Time. So important stories like Rumple and Belle, Hook and Emma, Regina and Robin Hood, Snow White and Charming and their baby — all those stories that we left last season are still very central to the storytelling of season four and will be explored further. Once fans can relax — we have no intentions of sidelining those stories for Frozen.

Can we expect nods to Frozen songs, like “Do You Want to Build a Snowman” and “Let It Go”?

Horowitz: Oh, for sure! We are huge, massive fans of that movie and we can’t help ourselves. There are nods to many things from that movie.

Kitsis: As we say, this is the most expensive fan fiction ever.

How would you describe Elsa’s entrance into Storybrooke? Chilly at best?

Kitsis: I think she’s a stranger in a strange land. Elsa is from Arendelle, and she’s never seen a car before. We know that when Elsa gets frightened, ice spikes appear — so imagine when she sees this world for the first time.

Horowitz: It’s sunny in Storybrooke, but she’s bringing a cold front. It is this mash-up of two worlds. She’s never been to our world and she’s never experienced what it’s like here, and our world has never experienced anyone quite like her before.

Now you have two queens in Storybrooke. Will one of the main conflicts this season be Regina and Elsa battling for power?

Horowitz: It’s not that it’s jockeying for power, it’s more about …

Kitsis: There’s a mystery.

Horowitz: And it’s hard to answer your question without spoiling, but I’ll try. When Elsa arrives, it signifies the start of a mystery that needs to be explored and answered. And the repercussions of that mystery reach out and touch everyone, and you’ll see it affects everyone from Regina to Emma to Rumplestiltskin.

At Comic-Con, a scene from the season-four premiere showed Regina recruiting an old friend, The Mirror, to help her get rid of Elsa. So who’s the biggest threat to Regina, Marian or Elsa?

Kitsis: The bigger threat in her mind is definitely Marian. We’ve shown three seasons of growth for the Evil Queen, and now she’s really faced with her first emotional setback, which is she thought she had her happy ending. And when you go back and look at the finale last year, you see her and Robin Hood coming to that party. For the very first time on the show, [Regina] really looked happy, and that’s been taken away. So for us, it's a now what? The old Regina would immediately rip everyone’s heart out. Will this Regina revert to heart wrecking, or will she evolve? And we’re going to find that out quickly in the premiere. But Regina is also going to be launched on a new quest with Henry this year. We’ve seen Henry in Operation Cobra with Emma, but we’re going to see a new one with Regina that we’re very excited about. And this is actually one that will transcend, although the first 11 will be Frozen, this is an arc that will go the whole season.

In the finale, Emma and Captain Hook seemed to be taking their relationship to the next level. Is season four their attempt at being a “normal” couple?

Horowitz: There’s no chance that Emma — the daughter of Snow White and Prince Charming — and Captain Hook will ever be a normal couple, but their feelings obviously reached a new boiling point at the end of last season. And we’re going to continue to explore those feelings, and we’re going to continue to explore how they can or can’t get closer.

Kitsis: But in a real way. Emma has had a lifetime of putting up walls, so one kiss does not mean love and [let's] go register at Pottery Barn. As for Hook, we’re going to see some old rivalries. We can’t forget that as much as he loves Emma and has done the right thing for her, we’ve seen him do horrible things in the past because he’s a pirate. He and Rumplestiltskin unfortunately are going to find their detente coming to an end.

How are Snow and Charming adapting to being parents to a newborn?

Horowitz: That’s also something we’re interested in exploring and showing. Being parents is a challenge and we want to, in a real way, show that. But it’s for them this strange thing of they’ve been parents before but they’ve never been parents at this stage. The last time they had a baby they put her in a wardrobe, and I think that’s going to affect everything, from how they parent baby Neal to how Emma reacts to their parenting and how they interact with Emma. It’s hopefully an interesting, new dynamic for all of them.

How deep in the doghouse will Rumplestiltskin be with Belle when she finds out the truth?

Horowitz: I would not want to be him when she does.

How will Michael Socha's Will Scarlet/Knave of Hearts be incorporated into the Once world?

Kitsis: We’re going to get a little taste of Michael in the first 11 [episodes] — glimpses of him. But Michael's character, as we remember, was a disgraced member of the Merry Men, and Robin Hood is very much alive living in Storybrooke. So we’re going to see them reunite. Obviously for people who watched [Once Upon a Time in] Wonderland, they are like, "Wait, he had his happy ending. He went off with the Red Queen." We are going to tell that story, what happened and what brought him back to Storybrooke. And that will probably be in the later half in the season, and it will be done in a way that if you’ve never seen Wonderland, we’re going to fill in the blanks. We’re also going to find out that he has an interesting connection to another character from the past, from season one of Storybrooke.

Horowitz: For those who never saw Wonderland, he gets his own introduction this season, and you’ll see how he’s folded into [the universe] …

Kitsis: But he’s at his core a thief and the sheriff, so they’ll meet in an interesting way.

Once Upon a Time premieres Sept. 28 on ABC.

Email: Philiana.Ng@THR.com

Twitter: @insidethetube