Kinoautomat

Film in the Czech pavilion at Expo '67 in Montreal, and was promoted as the world's first interactive movie. It was film-based, and all members of the audience had a red and a green button in front of them (the results of voting was displayed around the screen). The movie itself was a dark comedy about a man who believes he was responsible for his apartment building burning down, and is structured as a series of flashbacks leading up to the fire. After each scene the film would stop and a live performer would walk onto the stage and ask the audience to vote. Immediately, as if by magic, the voted scene was played.

How did they do it? Deceipt, of sorts. The branching structure wasn't tree-like, doubling the number of scenes needed at each choice, but rather always remained only two. They did this by carefully crafting a story such that no matter which of the two options were chosen, it would end up back at the same next choice. The vote was executed by the projectionist switching one lens cap between the two synchronized projectors. The artfulness, ultimately, was not in the interaction but in the illusion of interaction. The film's director, Radúz Činčera, made it as a satire of democracy, where everyone votes but it doesn't make any difference.

http://www.naimark.net/writing/trips/praguetrip.html
http://www.naimark.net/writing/badidea.html