 Runacre was born in Cape Town, South Africa.She moved to London as a child, attended the Actors' Workshop there, and trained in the Stanislavski System.While attending the Actors' Workshop, Runacre was approached by fellow student (and future agent) Tom Busby, who was working as a runner for an American film production that was seeking fledgling English actresses to play opposite John Cassavetes in "Husbands", a film to be shot the following year in London.