In horror movies, sich as the ring, there is a common effect that I have termed the "epileptic Spider". This is when someone is crawling on thw walls or ceiling, but the movements are choppy and unnatural. How is this effect best achieved?

I think you might be thinking of a speed ramping effect. If you don't do it in camera, then try speeding up small amounts of frames in post, perhaps chopping out a few here and there to taste. Coppola used this to great effect in his version of Dracula, worth checking out if you haven't seen it already.

Adrian, is it that simple?

The choppy movement part is either shot in camera with a variable framerate or as I said can be done in post by just speeding up a few frames and/or chopping some out. At least that's how I'd go about it, if I understand correctly the effect you're talking about?

The wall to ceiling part isn't so simple, I presume it would be either a tilted set or even a mechanically tilting set, not cheap!

I haven't heard of speed ramping being used for this effect, but one tried-and-true way to make an actor's movements appear strange and unnatural is to film the shot backwards. The actor performs their actions (walking, crawling, whatever) backwards and if there's a camera move it's similarly reversed (e.g. a push in becomes a push out). On a professional production shot on film the camera is run backwards, but on a digital production I'd imagine you'd just film it normally and reverse it with your NLE software.

This technique was used in Tarkovski's Solaris and Soderbergh's remake of that film, and I think I recall it being used in the remake of Ring but I haven't seen it in ages.

That's pretty interesting Nic. I'll have to keep that in the back of my mind for future use.

I haven't heard of speed ramping being used for this effect, but one tried-and-true way to make an actor's movements appear strange and unnatural is to film the shot backwards.

I've also seen this effect used with great results, especially when combined with the above mentioned speeding up on certain frames.