Bono had "no idea" Dolly Parton was close to death when U2 were collaborating with her on what would be her last ever recording.
U2's new album Carnaval de Luz features Parton on its closing track, Torn, a song that, following her death in August, has the distinction of featuring the country music legend's final vocal performance.
The Irish band's frontman says that while the group was aware she had health problems, they didn't know how serious they were.
"We knew she was ill, but we had no idea that it'd be her last vocal," Bono tells the BBC. "That was a real shock. A shock for us because we'd been working with her, but a shock for anyone who loves music."
Remembering how he felt after learning of her death, the Elevation singer adds: "I remember I was with Michael Stipe in a car when the news came in, and he actually fell down and started sobbing. He just could not believe it.
"Probably the only thing that people can agree on in the entire world right now is Dolly Parton. Whether you're gay or straight or conservative or liberal, she was ready for you to fly at her show."
Speaking about how the collaboration came about, Bono reveals that he sent her a set of bagpipes for her birthday and she agreed to listen to their demos and contribute.
"We had some folk songs we were playing, there was one called Lifelong, and I remember her saying, 'You know, that's a little too Irish for me'," the rocker remembers. "So she said, 'Look, I've started a song called Torn. You can take it where you want, and I might like where you take it'. And she did."
They then reworked the song with Snow Patrol's Jonny McDaid to address the political divisions tearing families apart.
Carnaval de Luz is out on 13 November.
