Virtual reality technology that puts you inside the movie.
Imagine a movie that has one extra character every scene. A character that is meaningless to the plot, never says anything and never moves out of his place. A different such character for every scene according to the plot. And that character is the viewer in 1st person participating more or less.
Imagine war movies where you are one of the soldiers. Your alter ego does things independent of your will but you see though his eyes and the main movie action is always nearby so you can participate.