Justin Timberlake spends "14 hours" at a time in the recording studio, according to British star Jessie J.
The singer-and-actor penned some tracks with Jessie J and the Do It Like a Dude singer says she was astonished by his work ethic.
She said: "When he's in the studio, he is in the studio. We would literally spend 14 hours in there and not leave."
Timberlake discovered Jessie J after witnessing some of her teenage performances on YouTube and she was overwhelmed by the praise he gave her.
Speaking to Glamour magazine she said: 'Justin sat me down and said, 'You have the most incredible voice I've ever heard''."
She added: "When someone likes that says it to you, you want to do one of those dance moves and sing, 'Love and sex and magic'."
Jessie J was originally signed to Sony in the US as a songwriter, and accepted the deal, unsure if she had a future as a performer.
She said: "I went round the UK and tried to get signed, and at the time it was like the bombardment of women - VV Brown, Gabriella Cilmi, Cheryl Cole, Adele, Duffy. Everyone was going, 'We're not going to buy into another girl now'."
"I got signed as a writer by Sony, so I flew to the US and went, 'Maybe I'm not an artist'. I knew at the back of my mind that I was, but sometimes you need someone to go, 'Come on!'"
