Andrew Watt has spoken with Guitar World about working on Ordinary Man with Elton in Atlanta.
The song appears on Ozzy Osbourne's album of the same name.
The producer and musician says that Elton asked him to make a chord chart.
''So I wrote one out and put it in front of him and he sat at the piano and played the parts. He went piece by piece as he learned it, and then he started going for full takes of the song.''
Asked how long the process took, the 29-year-old replies: ''He literally tracked his entire piano part in under an hour. It was pretty incredible.''
He adds: ''We're in the studio listening back to the song, and Elton's harmonising along with Ozzy's vocal. And it just clicked in that moment. I asked him, 'Would you mind doing some singing?' He said, 'Not at all.' And he took the second verse. It wasn't planned.
''And Ozzy loved it. Those words take on a whole new meaning when Elton sings them.''