You can use a PIR sensor to start world war 3. You have to divide your task into several sections. First you have your PIR sensor, which reads the spectator's position. Your PIR sensor sends information about that to next section. What is the information? The distance to the screen? Or also sideways movement? Or also which leg is moving? You might need more PIR sensors. The receiving device could be an Arduino, which only translates the sensor readings to say XY-coordinates, or just Y-coordinates. Arduino could send this as serial data to a computer, which in turn could create the animated person walking in the screen. Or have the computer start the prerecorded video sequences. Or start world war 3.