Better start warming up those vocal cords, Oncers!

Production of the highly anticipated musical episode of Once Upon a Time is well underway and executive producers Adam Horowitz and Eddy Kitsis are spilling exclusive new details to ET about what fans can expect when our favorite characters step into the spotlight.

Read on for inside scoop on how this episode will be a "huge part" of Once Upon a Time's mythology, the many romantic "surprises" in store for fans, and if Colin O'Donoghue will be singing!



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"We actually started [planning] this around September," Kitsis explained to ET over the phone last Friday. "There are these two composers that we met, Alan Zachary and Michael Weiner, who are big fans of the show, and ABC put us together with them because a musical is the one thing and the one request that fans have been asking for for six seasons."

"It's something we're very excited about and we've been working on for a long time," Horowitz concurred. "What we've also worked really hard to do is make this musical a part of the show, so it's not just like a one-off episode or it's not just like a stand-alone [special]."

You hear that, Oncers? The musical episode is not going to be some kind of a dream sequence or an alternate reality. It's going to be a "real Once Upon a Time episode" and a key part in progressing the storyline of the series forward.



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ABC

"It actually is a huge part of the mythology of the show and there are some big things that happen in the episode," Hororwitz revealed. "Frankly, it's been one of the challenges of doing the musical because we never wanted to do something where it was just like a to-the-side, one-off thing, and then get back to the main story. We want to see it part of the main story, which meant we had to really plan out this season with some great detail."

Kitsis added, "And what's fun about it is it's the penultimate episode [of season six,] so the musical actually kicks us off into the finale."

The musical, which begins shooting later this week in Vancouver, Canada, will feature seven songs including a solo from Jennifer Morrison's Emma. But here's the big question we wanted to know: Will O'Donoghue's Killian also be lending his singing voice to the set list?

"Colin will be singing," Horowitz confirmed.

(Deep breaths, y'all.)



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There were no details provided as to what exactly O'Donoghue will be singing, but when asked if a duet between Emma and Killian is something the fans can expect to see in the musical, the EPs were coy with their responses. "We want to keep some surprises for the fans," Horowitz shared.

"Yeah, but I would say this," Kitsis continued. "If you were watching a Once musical, we approached it like a fan. What would the fans want to see and who would they want to see sing?"

"Well, you know you never know," he teased when we pointed out that many fans would love to see a Captain Swan song in the mix. "It's a show about hope, Leanne, and if you have enough of it, it might happen!"

ABC

As for Emma and Killian's future, fans will be pleased to know that this second half of season six will "absolutely" feature many more romantic moments between The Savior and her swashbuckling leading man.

"What I can tease is that this second half will, probably, have the most romantic moment between those two in the six seasons of the show," Kitsis dished.

(Remember to breathe!)



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Many fans almost lost their minds when an engagement ring was featured in Once's midseason trailer, so we're curious to know if Killian might be finally be popping the question to Emma?

"At the end of the fall run of episodes, we teased that there was a ring," Horowitz established. To which Kitsis added, "So somebody's going to get it."

"But to be fair, Henry did just take Violet to the dance, so we don’t know," Kitsis quipped with a laugh.

New episodes of Once Upon a Time return this Sunday, March 5 at 8 p.m. ET/PT on ABC.

How are you feeling, Oncers? Are you excited that Colin O'Donoghue will be singing in the musical episode? Do you think a Captain Swan proposal is in our near future? Share your thoughts with @LeanneAguilera on Twitter!