Thrilled movie concepts

So sadly due to so much going on this week I was not able to post my concept so while I have a moment. Here it is.


The problem I have with most slashers, thrillers, and what not is you usually have a rough idea who the killer is, or it is already established early in the film. Also I always feel like the extra, the back space is being wasted. Sure you don't want to distract from the main focus but what if you utilized it?
Take the movie Scream and pick a random extra. They are the killer, through out the movie they appear what seems to be randomly in the background. Occasionally looking at the main characters or the audience, as a subtle yes it is me I am the killer. Granted this is merely a concept, but something that is not utilized. People would be tempted to go back and rewatch the movie and see that extra.
A simple scene could be late at night in school hallway or college hallway, the focus is on the couple talking as they walk towards the camera, in the distance is the killer dragging the teachers body or something. Movies tend to follow the same rules as DD, then world doesn't exist except for what is currently around the main characters. This concept would change that. Let the school be living and not static.