The composer Carter Burwell has a section on his website http://www.carterburwell.com , talks about the whole story behind "Bella's Lullaby," the song he wrote for Twilight in 2008.I!!!!! love it , love it !!!!
"Bella's Lullaby," as it appears in this film, was not written to be a lullaby but to speak of love - ecstatic, tormented love. Here's that story.
*Years ago I was in love with an amazing and challenging woman named Christine Sciulli. She left me, I was heartbroken, and I wrote a piece of music that tried to express the thrill and pain of having my heart pierced. She wouldn't speak to me, so I sent her the music to speak in my place.*
*Years later (April 2008 to be exact) I came to Oregon to meet Catherine Hardwicke and see some of the film she was shooting, Twilight. She mentioned that the producing company, Summit Entertainment, had just requested that a new scene be added to the film. The scene existed in the novel but hadn't originally been part of the screenplay (films are not one-to-one translations of books). In this scene Edward would play piano for Bella. I wasn't officially working on the film at that point and they didn't know what Rob Pattinson (Edward) should play during the shoot. This is not an unusual situation - many films have an actor sitting at a keyboard, swaying back and forth, pretending to play music that was only written after the film has been shot and edited. This case was unusual in that Rob is a fine musician, and fully capable of playing the piano or probably any other instrument.*
read the full article here : http://www.carterburwell.com/projects/Twilight.html#Lullaby_orig