William Theodore Kotcheff was born in Toronto, Ontario, Canada, to Bulgarian parents from Plovdiv. He graduated with a degree in English Literature from the University of Toronto. He began his professional career directing TV drama at age 24 at the Canadian Broadcasting Corporation, at the time becoming the youngest director in the CBC. After two years there he went to live and work in England, directing in television and the theatre.
Cassidy was discovered at age 14 by a talent agent in Florida and started modeling and acting in commercials. She got a one year scholarship at the Huntington Film Institute in Orlando, which led to a part in the Nickelodeon series Clarissa Explains It All (1991). She then (1993) moved to Los Angeles together with her mother Amy to start an acting career. Her first on-screen appearance was in the TV show Angel Falls (1993); next she was cast in the short-lived Spelling show Models Inc. (1994).