When i watched The Exorcist the first time it was in the west end i remember the atmosphere was chilling, i'm sure they used effects in the cinema like they did for the film Suspiria things like turning on the aircon full blast during certain scenes which seemed to be all the rage those days, There was a scene in Suspiria where a girl was swimming , the water was up to her nose and you could "feel" the water level on your face! It really worked to build up the atmosphere also feeling the wind blowing in your face during the corridor scene with the curtains blowing around the open window, talking of effects, who remembers the film Kentucky Fried Movie with the scratch card given out to audiences with the numbers coming up onscreen to tell you which panel to scratch and the dubious smells contained in themfitter said:I seem to remember reading a poll where the shower scene in Psycho was voted the scariest of all time.
1 or 2 from the Exorsist come a close second.