Director Paul Verhoeven used a unique trick to help the actors interact with an "invisible" person. He planted and had Bacon's voice move from one speaker to another, forcing the other actors to physically track the "invisible" character with their eyes and ears as he moved around them.
If you are interested in the film itself, Hollow Man is a science fiction horror film directed by Paul Verhoeven. It serves as a modern adaptation of the H.G. Wells novel The Invisible Man .
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