The couple fell out when they started taking singing lessons together for JOHNNY CASH biopic Walk the Line and it took them three months to become friends.
Witherspoon recalls, "It took us about three months to trust each other because we didn't know each other at all in the beginning and we couldn't even look at each other when we had to sing to each other because it was so embarrassing.
"I'd sing too loud and he'd say, 'It's driving me crazy, she's singing too loud. Does she have to sing so loud?' I said, 'I'm just trying here!'
"It took about three months before we responded to each other's work and saw improvement. It took a long time before we really felt comfortable with each other."
But once that happened, they became the best of friends: "There were days we felt bad. He would say, 'You're doing really good today,' or, on days he felt like crap, I said, 'Don't beat yourself up, you're doing a really good job.'
"We really leaned on each other and became close in that way."