Movie-maker James Mangold insisted both actors perform all songs themselves and they underwent "rock 'n' roll boot camp" to learn how to sing like the husband and wife.
And Witherspoon admits it took a while: "When we started we sounded bad. We were terrified someone would get our first recordings and put them on the internet. The longer we worked on it the better we got.
"The coaches got us to sing from our diaphragms and not our head, but it took six months to get to a place where I felt comfortable enough to record."