The Frighteners
Peter Jackson · 1996
The ghost scenes were shot twice, once with the human actors on set and once with the ghost actors against a blue screen. Actors had to deliver lines to empty air. When Frank talks to a ghost in a room full of people, the effect is that he looks strange to everyone but you.
Where to see it: Lynskey house job: Frank speaks to his ghost partners, who only he can see, while the living people in the room see him talking to nothing.