At its first performance in 1952, the `performer' sat down at the piano, and keeping track of the time on a stopwatch, turned the pages of the score, occasionally closing/opening the piano lid. Cage's idea was that anyone listening intently would have heard sounds, although not the ones they were expecting to hear.
Also relevant is Cage's `Reunion' installation. This made use of a photo-receptor equipped chessboard. Every time a player makes a move, a different sound is triggered.