Featured in the scene when Cassie discovers that Ryan is not who he says he is, "Pearl's Dream," taken from the 1955 movie Night of the Hunter, is meant to evoke the end of the fantasy for our heroine — the falling from the dream cloud.
After viewing the video filmed of her best friend Nina's rape, Cassie realizes Ryan was there. Her shock, disbelief, and terror are palpable — when she goes outside in a kind of trance, we hear the haunting lyrics: "Once upon a time there was a pretty fly / He had a pretty wife, this pretty fly / But one day she flew away, flew away / She had two pretty children / But one night these two pretty children / Flew away, flew away / Into the sky, into the moon."
The significance of this is the end of the innocent romance, the teenage-like infatuation with Ryan — the fantasy is over, and revengeful Cassie is very much back. Night of the Hunter's significance is clear, too: it follows a serial-killer preacher who targets women who use their sexuality to their advantage.