'I'm Just Ken' wasn't meant to be performed in 'Barbie' — until Ryan Gosling heard the song. Here's the story behind the epic dance sequence.

• "Barbie" is now available to rent and buy digitally, and includes new behind-the-scenes featurettes.
• Music producer Mark Ronson said the inclusion of "I'm Just Ken" was initially much smaller in scope.

Ryan Gosling's epic performance in "Barbie" might have been the highlight of the blockbuster movie, but it wasn't originally in the script.

In the climax to the highest-grossing movie of the year, which is now available to buy and rent digitally, the Kens face off in a beach battle to the tune of "I'm Just Ken," an epic 1980s-esque power ballad performed by Gosling's Ken.

Since "Barbie" hit theaters in July, the song has become a massive hit and even entered the Billboard Hot 100.

But the big song-and-dance number almost didn't happen, per a new behind-the-scenes featurette released on Tuesday alongside the movie.

In a bonus clip titled "Musical Make-Believe," "Barbie" music producer Mark Ronson said that "I'm Just Ken" was initially meant to be played in the background of a scene, rather than performed in the movie.

"It went from being this song that was maybe going to be played somewhere in the film to this song that they built this epic battle around," said Ronson.

In the clip, director Greta Gerwig explained that she asked Ronson to write a song for Ken after hearing the song he wrote especially for lead character Barbie, played by Margot Robbie.

"I instantly knew what the Ken song should be," Ronson went on to say.

After Ronson turned in a shorter version of "I'm Just Ken," he recalled: "Greta's like, 'We love that song but actually could you make that longer?' So we're essentially scoring eleven minutes just around the Ken song."

In July, Vanity Fair reported that Gerwig made "I'm Just Ken" a much bigger part of the plot after Gosling heard the song and said he wanted to perform it in the movie. Gerwig acquiesced Gosling's request and rewrote a scene toward the end of the movie to include his performance, the report said.

The dreamlike ballet sequence that is interspersed in the Kens' beach battle was entirely Gerwig's idea and inspired by 1950s musicals.

"I've always loved dream ballets. Like, in the middle of something that is happening, all of a sudden it is a dream ballet," said Gerwig in the featurette. "They do it in 'Oklahoma!' — it's an incredible dream ballet. They do it in 'Singin' in the Rain.'"

We had one day to get the Ken dream ballet, but we got it and it was really triumphant. And everybody felt good because, honestly, what's more fun than dancing?" the director continued.

Visually, the dance sequence is a direct nod to a similar dream ballet scene in "Singin' in the Rain."