Whistle while you work….

When you wish upon a star….

Let it go-o-o-o-o-….

Many a Disney fairy tale has been punctuated by memorable tunes, but ABC’s Once Upon a Time has never added music to its mix. That is, not until next Sunday, May 7, when Season 6’s antepenultimate hour, “The Song in Your Heart,” is cued up.

FROM JAZZED TO… JAZZ HANDS? | Since Day 1, nary a Once fan event has gone by where the cast and or creators have not been asked about the possibility of a musical episode. “Whenever they ask us at conventions, I’m always like, ‘Yeah. Let’s do it!’ — and I rile the crowd up even more because I think it’s a f–king brilliant idea,” Rebecca Mader, who plays Zelena, told TVLine during filming of the landmark hour. “So when I found out they were actually doing it I was like, ‘Oh my God.’ I knew that the fans were going to go nuts.”

Yet though the idea has oft been bandied about, having it become reality triggered a variety of reactions from the cast who would be asked to croon on camera.

“I said that in theory I was keen,” Ginnifer Goodwin shares, “but the truth is when it became a reality I grew terrified, and asked if I could be a doo-wop girl in the background. Jazz hands and all!” Lana Parrilla meanwhile said that on a scale of 1 to 10, her anxiety level started out at “maybe a 7 — mostly because I haven’t done a musical since I was 19 years old. I love singing in the shower and in the car, and with Rebecca — we’re always singing together [on set] — but I didn’t know what kind of number I would have. I didn’t know who was singing, if it was Regina or the Queen, and that really made a big difference.” (More on that in a bit.)

On the far calmer end of the spectrum were Jennifer Morrison and Colin O’Donoghue — which is, well, good, seeing as Emma and Hook’s wedding is a focal point of the musical hour! “Colin and I were predisposed,” Morrison says, “because we both came from musical backgrounds, so to us it just sounded like it would be really fun. We’re happy that it finally worked out.”

TV son Jared Gilmore meanwhile knew precisely the extent of the vocal skills he himself would bring to the table. “I have one singing line in the episode,” he reports. “At first I was kind of like, ‘OK. I don’t have a number,’ but then I was like, ‘Oh wait. It’s probably for the best, because I can’t exactly sing.’ I’d probably just embarrass myself. And my family!”

“I, for one, never thought we would do a musical episode,” Josh Dallas shares. “I know there was talk about it, and people would ask, but I just never thought we would ever do one.” But at the end of the day, he says, “Once Upon a Time absolutely lends itself to a musical format.”

MAKING BEAUTIFUL MUSIC TOGETHER | Although at least one cast member assumed that if this day came, Once Upon a Time would recycle famous Disney tunes, that was never the plan. Instead, series creators Adam Horowitz and Eddy Kitsis last summer sat down with songwriters (and Once fans) Alan Zachary and Michael Weiner to hash out the grand plan, which would lead to seven original tunes being penned. “After we let the cast know [about the musical episode],” says Horowitz, “they got together with the composers, to tailor all the songs to each actor and their character specifically.”

As a result, “They ended up presenting us with these gorgeous, original and really catchy pieces,” Goodwin raves. “I really can’t see them not being hits.” Morrison, who has a solo, concurs, saying, “They wrote the songs for our voices, because they knew our ranges and where we were comfortable.” As O’Donoghue puts it, “Just as the show takes these characters that everybody knows and makes them fresh and new, in a sense that’s what’s happened [with the music].”

Now, for the million-dollar question asked of every TV series that braves a musical episode: Why is everyone suddenly singing?

Simply said: We learn in flashbacks that Snow White, when pregnant with Emma, inadvertently cast a spell that makes highly emotional people only able to express themselves through song. That allows for musical numbers both in the Enchanted Forest of yesteryear as well in present-day Storybrooke, when the spell resurfaces.

NAME THAT TUNE | As for what you will hear once May 7 rolls around, there are seven songs in total. In addition to Emma’s solo (about “a song that has been living in her heart all along”), viewers can expect a highly entertaining “sing-off” between the Charmings and the Evil Queen (“Our weapons are our song,” Dallas previews), as well as what Mader describes as “a full-on Broadway number” for Zelena the Wicked Witch. “It’s almost like Once Upon a Time‘s reimagined version of [Wicked‘s] ‘Defying Gravity.'”

During TVLine’s set visit, the choreographers revealed that a number set aboard the Jolly Roger in a tavern features some very fun and fancy footwork, while the Evil Queen has a high-energy, slide-on-her-knees performance of a tune whose message is “down with love, down with hope,” Parrilla explains. “The Queen has moves,” she adds with a wink. “She’s a rock star!”

The song list also includes a duet that swells into a group performance, affording all of the characters on hand a chance for a whirl, a twirl or, in Gilmore’s case, a brief warble (his aforementioned “one line” elicited the sweetest of reactions from the crew, take after take after take after take). The rousing, climactic number “is so perfect for the show,” O’Donoghue observes.

(As for Rumple and Belle, due to spoilery happenings in the April 30 episode, titled “The Black Fairy,” they are “laying low” during the musical episode, says Kitsis, though Rumple will be seen.)

‘IT FELT LIKE THE PILOT AGAIN’ | The cast’s abundant enthusiasm did not go unnoticed when the show bosses arrived on set during production of the musical hour.

“It was really re-energizing,” Kitsis says of the endeavor’s infectious effect on both cast and crew — no small feat this late into the season, and even more impressive this deep into the show’s run. “It was Episode 130 and you really felt like it was the pilot again,” Kitsis says. “Everyone was nervous and giddy and excited….

“It was one of those things we were terrified of as showrunners, but once we saw how excited everyone was to do it, it made us really glad that we attempted to pull it off.”

Will the Once audience in turn respond with a standing O? “They’re going to love it,” Parrilla predicts. “To see these characters that they’ve been watching over the years sing is refreshing. It’s a totally different form of expression. And it makes sense, which I think is critical — that it really works with our storytelling.”

COMING SOON: Jennifer Morrison and Colin O’Donoghue open up about Emma and Hook’s big day: “It’s all happiness” — except for one looming threat.

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