The actress, who showcased her musical ability in movie-musical Mamma Mia!, has long harboured dreams of becoming a singer/songwriter.
And she was delighted when Lasse Hallstrom, the director of Dear John, asked her to bring her guitar on set to write a song for the film, which culminated in the track, Little House, being included on the soundtrack.
She tells Britain's Glamour magazine, "It was a happy accident. The director asked me to bring my guitar on set and I ended up singing a song that I'd written, which ended up on the soundtrack. Writing songs is a stress-buster for me."