How did you react when you heard about the project?
James Mangold, the writer and director, approached me two years before he even had a script. He said, "I'm going to make a film about Johnny Cash," and I thought my eyes were going to pop out of my skull because I'm from Nashville, Tennessee, which is sort of the centre of country music in America. So not only did I know everything about Mr Cash and his life, but I knew everything about June Carter Cash and the entire Carter family. They were the foremost country musicians of our time. I even played Mama Maybelle Carter in a fourth grade play. I was very excited when he said he was making this film.

So this was quite personal to you?
Absolutely, it was very personal. We shot in Memphis, Tennessee, in the middle of the summer, and it was one of the places I grew up in. I just knew the music so well and I knew the feeling, the attitude and the culture, which is a huge part of getting into a character. I look at where they're from, what is their orientation towards family, or away from it? What is their spirituality? So a lot of these were just very easy connects for me. And then there was the whole challenge of singing and playing an instrument and giving live performances - that was frightening but completely exhilarating.

Were there any parallels between your life and the experiences of Johnny and June?
To play June, I had an immediate connect with her background and culture. We grew up with the same religion and in a lot of areas with the same sort of values of family and spirituality. But I was really so inspired by what a modern woman she was. She was married to two different men, she had children by both of them and she was divorced twice. She was also on the road and one of the only women travelling with Jerry Lee Lewis, Elvis Presley and Roy Orbison. So she was a pretty tough cookie. She had to hold her ground and they had so much respect for her. So it was a really informative character to play.

Is there a danger when researching someone you hold in such high-esteem that you may find out something that might disappoint?
This was actually one of those great things. I didn't know that much about her specifically, and I don't think a lot of people do. Even people that know Johnny Cash's music really well and know that he was married don't really know that much about June Carter. In the Fifties, she was a huge star - she was a great comedian and this travelling woman who sang with every major male star out there. Every single one of them had a giant crush on her and she just would not give them the time of day. So she was just a great character to find. I said to myself, "She's going to be so much fun." Apparently, John and Elvis had a big fight over her. She had a whole thing with Elvis where he was completely infatuated with her and possibly something happened and that was sort of winked at to me by the family. John just became so jealous with rage every time his name came up.

The chemistry between you and Joaquin is really strong. He has a reputation for being very intense.
The rehearsal process was really helpful because we spent every day together for six months in the same little house, rehearsing, singing and practising our instruments. There were days when I was frustrated and he would support me, and there were days when he wanted to storm out the door and I'd say, "Don't go, keep trying." So we built a really strong friendship without anyone being there. It was basically just him and I and T Bone Burnett. So by the time we got on the set we'd known each other for six months and had a really close friendship. He's a great guy and I really respect him. He had so much passion to play that character. He'd go home every night and keep rehearsing and playing his guitar, or reading more books. He was really, really inspiring.

How difficult was learning to sing and then copying someone else's style?
Well I started working on trying to sound like her from the very beginning. It was one of those things where I thought I was going to be fantastic. I listened to her CDs and sang in my car for probably about a month before I had a voice lesson or did any recording. I thought, "I'm so going to nail this!" Then we had an initial meeting with Joaquin, which was the first time he and I had ever met. He wasn't sure whether we should start yet but I was like, "Get me in there coach!" I could tell Joaquin was thinking, "I hate her."
Then I started to record, and when they played it back, I asked, "Who's that?" And when they said it was me, it was like, "You're kidding me! I'm much better than that. I don't hit all those awkward notes or make mistakes like that."
But it was and I immediately hit the panic button. I called my attorney four times in a row and asked him to get me out of this, because I was determined not to suck in this movie. After many arguments and discussions they finally made me go back and start working with a vocal coach. It took me five months to get to the point where I could actually hear the playback and it didn't sound like nails on a chalkboard. Really, it was that bad.