Elsa's unexpected entrance comes "in the middle of the most intense, heated moments these characters have been in yet," Adam Horowitz tells THR

Storybrooke is preparing for an early winter — and viewers should, too.

In a few days, Once Upon a Time begins its anticipated 11-episode Frozen arc with the debuts of beloved characters Elsa, Anna and Kristoff, and picking up immediately after the events of Disney's 2013 Oscar-winning blockbuster. Co-creators Adam Horowitz and Eddy Kitsis were noticeably eager for the arrival of Sept. 28, the show's season-four return.

"You spend months and months before a season starts in a bubble writing and producing these things, so we're excited to finally let people see what we've been working on and get it out there and let the ride begin," Horowitz told The Hollywood Reporter. "There's a part of you that loves that little secret that you have, but at the same time you want to show it off," added Kitsis.

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Horowitz said the Frozen arc, which he and Kitsis actively sought out, comes "right in the middle of some of the most intense, heated moments these characters have been in yet." He added that the season opener, titled "A Tale of Two Sisters," is "a curveball" — and an "emotional roller coaster" — for the Once heroes, one that "causes a ripple effect" for episodes to come.

Last Once left off, Elsa emerged through a time portal mistakenly left open by Emma and Captain Hook, leaving a trail of ice in her wake. Questions over who Elsa is (Once is currently set in early 2013, prior to Frozen's release), her motive for being in town and what she may be searching for are addressed in the first episode. (Another significant Disney property is also teased.) Even so, Once stars were enthusiastic about the melding of the two universes, with many saying it added a refreshing new layer of storytelling.

"Having Anna, Elsa and the world of Frozen come into ours has really opened [up] storylines and given us opportunities to explore different parts of our characters and stories we never would have had without them," Jennifer Morrison told THR. Her character, Emma, will embark on an unlikely friendship with the Queen of Arendelle, whose "disconcerting" arrival causes literal mayhem for the town of Storybrooke and an unwelcome nuisance in a menacing snow monster.

"Elsa becomes very important to Emma because they have so much in common," Morrison explained. "They're both people who have powers they can't completely understand and can't completely control, and they can relate to feeling rejected and misunderstood for most of their lives. This is the first time Emma has a friend who truly, truly understands what she's been going through."

Other characters from Frozen coming into the fold include the dastardly Prince Hans and his brothers, reindeer Sven and the CGI-animated Grand Pabbie, as well as a mysterious character played by Elizabeth Mitchell. "It's a seamless transition," Ginnifer Goodwin told THR. "Though we are not replicating anything from the animated movie because we're doing everything in the Once Upon a Time twisty-turvy way, it's going to really justify [Frozen]. They have so much to offer our characters and absolutely teach us a lot over the course of this half-season."

While Elsa and Emma may eventually become friends, Snow White will have a far different relationship. "There is a moment where Snow is confused by the woman in the blue dress standing in her apartment," Goodwin hinted, adding that her character will be on the outskirts of the Frozen mystery — at least in the beginning. "My character gleans nuggets of wisdom from Elsa that I find very powerful, but I don't yet have a direct place in their story."

Another local unhappy about Elsa's entry is the Evil Queen, and she has good reason to be wary. "Regina is semi-exhausted having to fight yet again. Like, 'Now what? Who is she?' " Lana Parrilla told THR. "She doesn't like newcomers and [Elsa]'s another problem. We just got rid of the Wicked Witch and now we have a new villain we have to deal with — or is she a villain? She's a mysterious character to us all and that's the journey that we see in the first part of the season."

Many of the core castmembers were immediate fans of Frozen — save for Once Upon a Time in Wonderland alum Michael Socha (Will Scarlet/Knave of Hearts), who admitted to never having seen it (though his young son has "and loves it!"). For Josh Dallas, what struck him most was the love between the two sisters, a connection he felt was as significant, if not more so, than the romantic kind. "It’s a great modern love story. Not all love stories have to be about princes and princesses," he told THR.

As for the actresses portraying Elsa and Anna, Georgina Haig and newcomer Elizabeth Lail, the anticipation is building. "We are portraying characters who are hugely loved and respected by many people and we want to do them justice. We hope we have," Haig told THR. Even with the unwieldy expectations, Haig and Lail are hopeful they'll showcase a unique side to the sisters.

"Anna does everything all-out. She's fiercely spirited; if she's on a mission, she's on a mission. She does everything with her whole heart," Lail told THR, key traits she's hoping to bring to Once. Added Haig: "Watching the movie, I thought, 'Gosh, she's got so much sass and she's so strong and fierce as well.' I wanted to get the balance right of someone who is their own worst enemy but still is powerful and confident sometimes, but not always. Like we are in life."

Expect "tons" of Frozen references, Lail teased. Just don't hold your breath for musical performances of memorable tunes like "Let It Go" or "Do You Want to Build a Snowman." "You have to do a shot for every reference to Frozen. You'll get very drunk!" Haig joked.

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

Email: Philiana.Ng@THR.com

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