I mean, for all horror movie on DVD's, you get to watch the
Special Features that usually include "The Making Of..." and scenes
taken from the filming process where the acting cast play pranks
and etc. That reinforces the fact that it's all fake and just
acting and I often imagine at the end of scenes, the director
shouts, "CUT!" and everyone bursts out laughing. 
I usually scrutinise these movies to see how 'authentic' they look,
and if things are anatomically-correct and realistically possible.
Sometimes, they can get quite funny esp when the blood is of a
different colour / consistency, etc.
For that matter, I think the folks doing the make-up and who
prepare the 'corpses' for CSI are doing a brilliant job! Sometimes,
I'm really awed at their special effects! 
