As a piece of musical theatre, Come From Away breaks a lot of rules.

By this point in the show’s extraordinary trajectory, the unlikely nature of its subject matter — the kind treatment of thousands of unexpected visitors by the residents of Gander, N.L., in the aftermath of Sept. 11, 2001 — is well known. So is the fact that it’s based on real-life testimonies of people involved in these events.

On a formal level, too, Come From Away diverges from some musical theatre conventions, starting with its length. While there are notable exceptions (one being The Drowsy Chaperone, another Canadian-made hit), musicals tend to have a two-act structure. Come From Away runs in a single, 100-minute act.

And it’s also the case, as noted by director Christopher Ashley — who won a Tony Award for the show — that it has “no bad guy or central character.” Twelve actors switch back and forth between playing residents of Gander and the “plane people,” with no individual emerging as hero or villain.

Hamilton creator Lin-Manuel Miranda recently took to Twitter to rave about Come From Away, pointing out that its success rests in exactly this unusual combination of structure and subject matter.

The show has very few “button moments” — points where the audience is cued to clap — which means that the “incredible kindness and generosity and beautiful work” that audiences witness in the show accumulates in a manner that Miranda praised as “incredibly effective” and “ruthless with your emotions.”

But the show wasn’t always structured this way, explained its co-creators, husband-and-wife team Irene Sankoff and David Hein, in an interview before this Sunday’s premiere of a new edition of Come From Away, with an all-Canadian cast, at the Royal Alexandra Theatre.

It began life as a 45-minute presentation at Sheridan College in 2012, and went through multiple development phases at various theatres and festivals. The first full-length version, performed in 2013, had two acts, with the intermission coming after the big party number “Screech In” and before the showstopper “Me and the Sky.”

The idea to merge the two acts came from the show’s New York-based producers, Junkyard Dog Productions, who came on board in 2014. They pointed out that “no one who arrived in Gander got to stop the experience and step out and say, ‘How do you think it’s going?’” says Hein. “It was a continued experience and needed to go from beginning to end.”

While some producers might push for an intermission to sell drinks and merchandise, Junkyard Dog “pushed artistically because this was the right choice for the piece,” Hein adds.

This meant cutting material, which Sankoff and Hein say was challenging but was also a constant push-and-pull throughout the show’s development, which they characterize as a team effort between themselves, director Ashley, choreographer Kelly Devine and music director Ian Eisendrath.

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Making cuts was sometimes Sankoff and Hein’s choice alone, and sometimes came out of suggestions from Ashley. “Chris would be like, ‘Take a look at that . . . I know you’re used to it, but look at the storytelling without it . . . what do you lose and what do you gain?’ And then we go away and we rant and rail, and we wake up and are like, ‘Oh God, he’s right,’” says Sankoff.

A beloved number that they just couldn’t fit into the show is called “Washington.” It came from a story that Claude Elliott, the mayor of Gander, told them about going to America’s capital to receive a community resilience award on the 10th anniversary of Sept. 11 and feeling out of place.

“Afterwards the people onstage told him that their story meant so much to them, and thanked him and Newfoundlanders for giving them hope, and he realized the story (of Gander’s involvement in Sept. 11) was important,” says Hein.

They wrote a song telling this story and at one point tried to open the whole show with it, turning the rest of the action into a flashback, explains Sankoff. “I suggested that like 17 times and everyone . . .”

“Like a lead balloon,” continues her husband.

Then they tried to put it at the end of the show, as the action moves forward to the years beyond Sept. 11, “but it came too late in the show to have a focus on one person’s experience . . . it stopped the momentum,” says Hein.

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This and a number of other cut songs exist in rough demo form. Sankoff and Hein say they hope to record them and share them with audiences someday.

“We call them ‘Cut From Away’ songs,” says Hein.

And then there’s the song that Toronto audiences haven’t heard yet.

The character of Hannah, the mother of a missing New York firefighter, was somewhat sketchily drawn in the first Toronto run in late 2016, because Sankoff and Hein had heard about the woman who inspired her story but hadn’t yet met her.

That December, they finally went to her home outside of New York and were surprised to find themselves at a big family dinner. “We were welcomed in like we were when we went to Newfoundland,” says Hein.

“Like they were cut from the same cloth,” says Sankoff.

They were still writing the song that resulted, “I Am Here,” as the company was recording the cast album.

“We ran into the recording studio with pieces of paper and sticky notes and threw them in front of Q. (Smith, the actor who originated the role of Hannah),” says Hein. “It was in the last half-hour of the entire session and we recorded this entirely new song.”

The number was inserted into the Broadway show and features in the new Canadian production opening this week at the Royal Alex, following a run at the Royal Manitoba Theatre Centre in Winnipeg.

With the musical now running in two places (and a third touring company starting up next year), Sankoff and Hein’s next project is . . . more Come from Away.

When we spoke in December they were in the thick of writing the screenplay for the movie version of the musical. If it sounds like they’ll never escape this material, Hein says they’re happy with it.

“We’re insanely lucky and honoured that we get to spend our lives telling the story of Come From Away and sharing Newfoundland with the world. . . . It’s a real gift to wake up every day and say, ‘We get to talk about people being kind to each other.’ It helps with your news feed every day.”

For tickets and info on Come From Away, visit mirvish.com or call 416-872-1212 or 1-800-461-3333.