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The Disney hit “Frozen,” considered a near-lock to win the best animation Oscar, was not always the story of two feuding but loving sisters.

A major turning point came when Robert Lopez and Kristen Anderson-Lopez, the husband-and-wife songwriting team who composed the musical numbers, delivered the anthem “Let It Go.” Performed by Idina Menzel, who voices the older sister Elsa, as a moment of self-acceptance, it’s now being YouTube-parodied by fans everywhere, and is also an Oscar front-runner.

But the movie was actually a years-long series of small turning points, as the Lopezes, who live and work in Brooklyn, described in an interview this week.

When they were first approached to work on the film, after having worked on “Winnie the Pooh” for Disney, they were enticed by the storyboards. “We saw this pictures of two sisters, and one of the sisters was throwing snow up in the air, and the other was delighted by it, and something about that really resonated with us,” Ms. Anderson-Lopez said.

But a director, Chris Buck, told them that “the rest of the story was in flux,” she recalled. “And we understood, after we saw a table read, why this was true. Most of the characters were really unlikable. It was an adventure – ‘Romancing the Stone,’ ‘Raiders of the Lost Ark’ kind of feel.’”

They wrote some songs, but it quickly became apparent that the movie, as drafted, wasn’t meant to be a musical. Disney reshuffled the people on the project, bringing in Jennifer Lee, who had been working on “Wreck-It Ralph,” as a writer. Ms. Lee eventually wrote the script and co-directed “Frozen” with Mr. Buck. A week after she joined, Ms. Anderson-Lopez said, they had a story retreat at a hotel in California. “That’s when we really had to go into the DNA in the story and say, ‘What kind of story sings?’” she said. “That involves a big change in your main character. Your main character has to have some optimism, and she has to feel things so deeply that she will burst into song.”

That character became Anna, the feisty younger sister, voiced by Kristen Bell. “At the time Anna was this perfect, perfect princess,” Ms. Anderson-Lopez said. “And Elsa was really just jealous.”

To reconfigure Elsa from her villain role, they wrote “Let It Go.” “When we first penciled it in,” Mr. Lopez said, they called it something which, to our chagrin, we cannot print here. “Elsa’s song of empowerment,” Mr. Lopez suggested as a replacement. “Elsa’s fierce song,” his wife added. Mr. Lopez: “Elsa’s kick-butt song.”

(Especially accurate since the script at the time called for Elsa to come down from her mountain hideaway and attack her village, Mr. Lopez said, “with her army of snowmen.”)

But the Lopezes saw the sisters – a Disney first, in terms of lead heroines – in a different way.

“I said, ‘You have a chance to make the first really funny Disney princess,’” Ms. Anderson-Lopez said. “I’m so exited about the potential that Anna has to sort of bring in the world of Amy Poehler and Tina Fey, the goofy self-deprecating female heroines that are in our culture” now.

For inspiration, they to music that wasn’t Disney either. “We listened to a lot of Adele, and Katy Perry and Avril Lavigne,” Ms. Anderson-Lopez said. “And we listened to singer-songwriters – Aimee Mann, Tori Amos – the more tortured singer-songwriter who talk about keeping secrets and having things that they’re fearful and they’re ashamed of.”

And then they took that fateful walk in Prospect Park, where they came up with the song. Ms. Anderson-Lopez had already pitched ‘let it go’ as a hook to Disney – “it was about letting your past go and also letting your power go,” she said – and as they strolled, they thought about a character whose goals of perfection limited her.

Ms. Anderson-Lopez said said she was thinking about what it was like to be a female artist “and how you often have to stop worrying about what people think of you and wanting to be liked, in order to do your best work. We started walking and riffing on things, and thinking from an emo kind of place.”

Emo is not a quality Disney is known for. But then, sweeping, sweet Disney numbers are not what Mr. Lopez – a Tony winner for the biting musicals “Avenue Q” and “The Book of Mormon” – is known for, either.

“As soon as we started writing, I knew it would be a stretch for me, because I usually write songs that make fun of these kind of songs,” he said. “But when you do a Disney movie, you’re going to write emotion — that’s sort of the cue. Part of the reason I was attracted to the movie was because I knew it would stretch me emotionally. I wanted to write a ballad. But this really totally trumped all my romantic ballads. This is so cool to have written a song of empowerment, for girls like my daughters.”

They were 6½ and two½ when their parents started working on “Frozen,” and were the first to sing “Let It Go” incessantly around the house.

Ms. Anderson-Lopez, whose theatrical credits include the Off-Broadway a cappella musical “In Transit,” set in the New York subway, also saw her mission as a message to her daughters. “I was really excited to write an anthem that said, ‘Screw fear and shame, be yourself, be powerful,’” she said. “I wanted to give them the same message that I’m always saying at bedtime: ‘it’s O.K., you don’t have to be like everybody else.’”

As Oscar hopefuls, the Lopezes are occupying rarified territory – particularly Mr. Lopez, who has already collected an Emmy and a Grammy along with his Tonys, making him on track to be a young EGOT winner.

He won his first Tony, for “Avenue Q,” at 29. “I never thought we’d equal that experience – wining that Tony was bizarre and off-the-charts weird,” he said. “I’d been telling myself to never expect anything to live up to that experience. And then ‘Book of Mormon’ came along, and then I lowered my expectations for the rest of my life. And then ‘Frozen’ trumped that experience. And now I’m telling myself that nothing will ever be as good as this.”

But the couple has not deviated much from their Park Slope life. “In the middle of all this, we adopted rescue cats that have really bad parasites,” Ms. Anderson-Lopez said. “That’s been keeping us real: cleaning the cat vomit off the wall.”