Its director “Love Actually”Richard Curtis, and the songwriter and musician Ed Sheeran they have known each other for many years. Sheeran’s first girlfriend was the babysitter for Curtis’ children.

Since then, they have collaborated on several projects, including Comic Relief and the 2017 Danny Boyle-directed film Yesterday in which Sheeran stars and Curtis co-wrote.

So when the director was planning his first animated project “That Christmas” he showed the singer some early storyboards and the discussion came down to whether it would be a musical.

“He asked me if I wanted to write the music for it and I said, ‘Fine.'”Sheeran said from his home in London. “There was a scene and I wrote a chorus for it, but I didn’t hear anything” he explained to Variety.

Two years later, Curtis asked Sheeran to finish the song “Under the Tree” and briefed him on the story.

The “That Christmas” is available now on Netflix, it’s no longer an animated musical, but rather an animated film based on Curtis’ trilogy of children’s books. Set in the fictional British city of Wellington-on-Sea, it follows a series of stories about family and friends, love, loneliness and Santa Claus making a big mistake.

While Sheeran has written Christmas songs, he had never written a sad one until this moment. “It’s the only thing I wanted to write,” the singer said. “I’d never seen the need to write a sad Christmas song until I wrote this one…it’s a lot of people’s reality at Christmas.”

The songwriter put himself in the position of waiting for his dad to come home for Christmas – to no avail. And while the film itself is upbeat, the scene in which the song is heard is “heartbreaking”.

After Sheeran delivered the song, he thought it was over. “Netflix edited the song into the movie. I told them, ‘Look, I don’t do anything this time of year. I’m basically just with my family.'” he reported.

However, he set a condition. If they wanted a music video, he would do it if Curtis was going to step in and direct, even though the director had never made a music video. “I felt like putting his stamp on a Christmas song would be something special for me.” Ed Sheeran said.