I thought I’d post progress here as I’ve a few interesting bits to share so far. So, my idea was to use a Pi with a wide angle night vision camera to detect the motion of passers by and use this to trigger a jack-o-lantern to watch/follow them as they walked past. Hopefully in a kind of spooky/creepy manner! Then maybe I can add lighting effects/sounds too depending on how well that first part goes.
I started devising some kind of mechanism to sit my pumpkin on that would allow it to rotate. I think I’m going to keep the camera fixed (rather than mount it on the rotating part) as it should simplify the tracking part to start with.
I came up with this (CAD in Fusion 360):
The idea being that the scary-looking spikes will stick into the pumpkin to hold it in place, then the plate sits in a bearing and can be rotated by a stepper motor with belt drive pulley underneath.
About 32 hours of printing later I had the following. The bearing might be a little over spec as I think it’s a wheel bearing from a truck, but it works! 
:
Now to work on the software and electronics!
