In the "Angel" season one episode "Sanctuary", Faith forms a bond with the vampire-with-a-soul Angel; executive producer David Greenwalt explains Angel can help her because he alone is able to understand the suffering she has been through and how to help her atone for her sins. Faith is then able to return the favor in "Angel"'s fourth season, when she is the only one determined to defeat Angel's soulless alter ego Angelus without killing him in the process. Actor David Boreanaz explains, "I think having a character like Faith come back at a pivotal point when she finds out Angelus is loose is really, for her character, a way of saying: 'I'm paying you back, Angel, for saving me, therefore, I'm gonna save you.'" The writers believed it would be an interesting dynamic to have former "bad girl" Faith play a heroic role against the show's now villainous protagonist, Angel.