Basically, the argument goes that watching a film requires the interaction between the spectator and the images on the screen. When we watch a film, we are engaging in a cathartic experience, allowing us to release tension and through emotions. However, watching a film is also an overtly voyeuristic activity - if you think about it, when you got to see a film, you are effectively looking in on the lives and behaviour of the people on screen, whether it be a Ben Affleck/J-Lo love scene in a rom-com or a sorority girl being chased upstairs in a teen slasher movie.
When someone is placed in front of a screen and engages in watching a film, they are referred to as the spectator. There are 2 types of spectator:
Active: a viewer who doesn't merely watch the film, but engages with and makes meaning from it by consuming it and is encouraged to formulate an informed response about the film afterwards.
Passive: the opposite of this. Someone who is not engaging wholly, is just watching for pure enjoyment and is controlled by the overwhelming mechanisms and physical prescence of the film screening.
As the spectator, we are conscious of shared reactions, ie. laughter, which can alter the individual's response. For example, if other people in the cinema auditorium laugh at a comic moment in a film, then you, the individual specatator, may laugh too, in order to conform with those around you. When we watch a film, we are aware of the audience and are constantly checking our behaviour against that of other audience members to ensure conformity. In becoming an actual audience member, we have an individual and collective sense of our actions - although we become ingrossed in what's happening on the screen in front of us, we are constantly self aware.
However, spectatorship theory extends to beyond the cinema auditorium. After the film, we may engage in another expression of audience membership as we discuss our reactions and opinions in a variety of contexts, for example in the pub or on the bus coming home with friends after the movie.
Spectator vs audience - we are constantly checking our reactions against those of others to ensure conformity.
Watching a movie is both a voyeuristic and cathartic experience...you get more for your money that way :D