I have found that with each project I have a "go-to" song that becomes my theme song for the book. For my first novel, called SECOND SOUL, which was loosely about a boy who learns his father is possessed by a powerful demon and to save Heaven from the demon he has to kill his father. Well the demon ends up slipping from his father and into the boy's girlfriend. The book ends with the MC, a boy named John, looking into the Fall of Man (a fortune telling waterfall within Heaven) and seeing his girlfriend bearing the tell-tale white scar of the demon he thought he had killed when he murdered his father. So of course, my theme song was Lady Gaga's Bad Romance.
Now, that book was my practice novel so it'll never live outside of my computer. But it started a ritual for me--every novel must have a theme song. It's my "go-to" song for the book that no matter what brings me back into the story.
For GRAVITY, my song is this:
So tell me, do you have a "go-to" song?