The faces are actually facing INWARD, not outward. The square flush facing material around each face is lit from behind, and the material must be translucent so it casts a shadow which can be seen from the front side.
hope that helps... very cool effect.
after a closer observation, the whole panel must be made of a translucent plastic or material somehow, and the shadows are cast from a light source hidden behind the "frames" around each face.
How to replicate it yourself? that is an EXCELENT question... you would have to create a mold somehow, but use a material which passes light like this does. I could imagine a green or yellow light, MAYBE even blue, but it would have to be pretty bright regardless.
It's an old trick but a good one. They are vacuum formed faces. After you have your forms you mount them concaved, inward. Then light them from behind. Great effect, always loved them at the haunted Mansion (I have been at Disneyland for the past two days enjoying this very effect!