First I made sculptures out of Sculpey modeling clay. The busts measure 17" tall by about 8" wide.
Then, I had a friend of a friend, use a vacuum-form machine to make the positive/negative form. The process basically works by taking a thin, hetaed sheet of plastic, draping it over the sculpture then placing it on a machine-table that has a whole bunch of tiny holes in it that's connected to a igh-poered vacuum. It sucks the air down causing the heated plastic to form around and in the shape of the sculpture. My friend had to re-cast my Sculpey models in plaster because the pressure of the vacuum would crush them to pieces. They power on the machine used wasn't great do it didn't quite capture all of the detail I put in but enough to make an impression.
The effect is basically that the busts seem to turn to follow the viewer's every movement.
Here's a photo of how the effect basically works from "behind the scenes". The viewer is actually looking at the bust/face from the reverse side. A light is placed on the other side to help light and see the details. I seem to remeber reading somewhere that the Disney imagineers discovered the effect while making cast for the 'Great Momemts with Mr. Lincoln' show.
Here's what they look like (using a Night-vison on my video camera).