The film is almost totally improvised. The film's 'bible' was only 'bare bones' - i.e., driving a car, while videotaping a dialogue of the trip.
Second thought: This movie brings me back to the key question posed in THE INVITATION: Is he L.A.-crazy or crazy-crazy? (CREEP was shot in Crestline, California, in the mountains northeast of Los Angeles. The heart rock is real, visible on one of the main hiking paths from Camp Seeley.
