No one writes a play just to have a bunch of people say lines and move around. The play is written for the audience. Everything that happens in a production is calculated to get an anticipated response out of the viewer. While we, as spectators, have no lines to memorize, or places to stand, there is a part for us to play. It isn't written down and rehearsed, but it is implied and anticipated for us to be taken in and to feel involved in the play or movie.
When we watch a play production, like A Winters Tale, that I had the opportunity to view recently, the audience and viewers can interact with the actors, and can influence the production. by the responses we give, laughter or booing and so on. The actors can even interact with the audience, say hello to us, throw things at us, grab us and dance with us, whatever.
When we view a movie, we play an incredible character. A role that the director forces on the viewer. One that he tries to manipulate to feel certain things and think a certain way. We don't say anything, we can't influence the story in any way, but we are given the role of a powerful avatar that can be anywhere, that is invisible, sees everything, can hear thoughts, can hear music that isn't really there, won't die (what if someone made a movie where the viewer died?). The viewer can be right up in the face of a character (which in real life or a play production would be really awkward and would not produce the same effect) and really see the nuances of their facial expressions and hear the quiet whispers they speak.
In every production we have a part to play, in a movie, a play, or a book even, we get to be a fly on the wall, we get to become someone else in a story that is not ours, and play a part that is written somewhere, but not on the script.