lets-go-to-the-movies:

For a dose of authenticity in the final twist at the end of the film, the actors were not told, prior to filming, that Geoffrey Rush would be appearing in the movie. They were told, before the scene was shot, that the person coming down the stairs would be Anamaria (Zoe Saldana) from Pirates of the Caribbean: The Curse of the Black Pearl; the looks of surprise on their faces as Rush descends are genuine.