Katherine Evelyn Anita Cassidy is an American actress. Following several minor television roles, she came to attention as a scream queen after starring in the horror films When a Stranger Calls (2006), as Kelli Presley in Black Christmas (2006) and as Ruby in the third season of the horror series Supernatural (2007–2008). Following a supporting role in the action film Taken (2008), Cassidy played leading roles in the slasher series Harper's Island (2009) and the remake of the drama series Melrose Place (2009–2010). She starred as Kris Fowles in the slasher film remake A Nightmare on Elm Street (2010) and had a recurring role as Juliet Sharp during the fourth season of the teen drama Gossip Girl (2010–2012).

When a Stranger Calls is a 2006 American psychological horror film directed by Simon West and written by Jake Wade Wall. The film stars Camilla Belle, Brian Geraghty, Katie Cassidy, Tessa Thompson and Clark Gregg. Belle plays a babysitter who starts to receive threatening phone calls from an unidentified stranger, played by both Tommy Flanagan and Lance Henriksen. It is a remake of Fred Walton's 1979 horror film of the same name, which became a cult classic for its opening 23 minutes, which this remake extends to a feature-length film.