Ed Sheeran's 'Fault in Our Stars' Song 'Means So Much' to Author John Green

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Shailene Woodley and Ansel Elgort

“He’s amazing,” Green says of the flame-haired Brit, who performed "All of the Stars" on Wednesday in a YouTube livestream.

John Green may have created Hazel Grace's world, but when it came to her soundtrack -- he was clueless.

"The only band I listen to is the Mountain Goats," the Fault in Our Stars author said on Wednesday. "The last time I heard new music was like, 1996. I’m a very old person." (For the record, he’s only 36.)

Enter: director Josh Boone, who, together with his team of producers, selected an on-point roster of artists to contribute to the film's soundtrack. Among them: Ed Sheeran, Birdy, Charli XCX, Lykee Li, M83 and Grouplove.

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"Now I own Ed Sheeran albums, he's amazing," Green confessed, shortly before Sheeran, Charli XCX and Grouplove were set to debut their songs in front of a live Los Angeles audience at the new YouTube Space. For all those at home, the event was livestreamed -- where else? -- on YouTube.

Unsurprisingly, Sheeran closed the show with his end credits song, "All of the Stars."

That the talented artist "would share his talent with this story means so much to me," Green gushed. "I’m so excited about that."

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For the 23-year-old Brit, he was tasked with guiding the audience from the intense emotions of the film, back into their own worlds -- a job not unlike the one he was given for The Hobbit: The Desolation of Smaug.

"I’m not writing about dwarves or dragons or dungeons [this time]," Sheeran cracked. "[It's] a tiny bit more relatable, a bit more human. A little bit easier to write."

Sheeran said his song was inspired not just by one scene, but by the film as a whole.

"It was kind of inspired by the whole movie, just wanting to be sad, yet euphoric and lift people a little bit, which I hope it does," he said. "I hope it doesn’t depress people too much."

Depressing songs were nowhere to be heard during Wednesday's event, with Grouplove performing their celebratory "Let Me In," which plays during a particularly lighthearted scene in the film, and Charli XCX rocking out to "Boom Clap."

"I feel like both my song and the movie are these ginormous love stories, and I think that’s why they click together so well," the "Fancy" singer told THR, explaining that her song was written prior to getting involved with the film. "I think that 'Boom Clap' was written from my heart and about this idea of epic love. I think that’s exactly what Gus and Hazel have in this movie."

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For anyone who's read Green's heartbreaking love story, tear-jerking moments are not only expected but desperately desired. For filmmakers, including producer Wyck Godfrey (Twilight, The Maze Runner), it required delicate precision to nail the film’s unique sad-happy tone.

"You are always walking that line and hoping not to be melodramatic," he said of working music into the film. "Sometimes score would overdo it, so we would pull the score out and put in a song that we could play under it. Sometimes an instrumental song is better than a vocal song.

"You just want to walk that balance of never hitting people over the head," he continued. "Let them have their emotion [and] let the song support it rather than push it."

The Fault in Our Stars soundtrack hits shelves on May 19 via Atlantic Records, while the film opens nationwide on June 6. If you can't wait until then, Green revealed on Wednesday that theaters across the country will screen the movie on June 5 at 8:30 p.m. ET, to be immediately followed by a live streaming Q&A with him and the cast.