I've been startled by a movie, but never genuinely scared. I have yet to see the Ring (waiting for it on DVD).
Of all the movies I've seen, the one that made me feel the most genuine discomfort/concern for the characters/borderline fear was Signs, not so much because of the script or anything, that was pretty straightforward. No, it was the sheer genius of the camerawork.
We as moviegoers are usually privy to information, especially in horror flicks, that none of the main characters are. We know when they are ignorantly walking into danger, which makes us nervous/tense if the movie has pulled us in. Sings, however, took a stance very VERY unique to me, and it will probably remain unique for the rest of my life as the first film to do this. The effect was probably enhanced by the great character development in the film, and how well their strengths and weaknesses are played to the viewer to draw them into this tight-nit family. When you think about it, how many other characters did we know much about in this film? Hardly any. But I digress, what made this film so unique was the AMAZING camera work in the film. Never before have I seen a film where NOT knowing what was going on was used so cleverly to cause so much... almost TOO much, tension. I have never been in the position where I've been stretching in my seat to see around the corner of a doorway that's on the movie screen, because the main character can see it but I can't. Such a unique experience shall never be repeated.
Sorry if I rambled... 16-hour day, etc...