The film required different emotions so the two sisters had swap out faces and head cores that permanently stayed on their bodies. First, the sculpted heads have a seam line scored along the hairline and side of the face. Then it is molded and cast in plastic, and the faces are dremeled off so that the faces and back of the heads are separate. Keyholes and eyeholes are dremeled into the head core and the inside of the face masks are built up with Propoxy to create the "locking key" that would interlock with the holes in the head core. The inside of the face masks are also dremeled away so that the eyeballs are flush with the eyelids. Then the parts are molded and recast again in plastic, now as separate head cores and face masks, since it would be easier to touch up the seam and keys if the parts were completely made of hard plastic (as opposed to thin layers of built up Propoxy). The molds are also made with better vent paths so that the face masks can be cast more smoothly. The clean up was done with Bondo.
Because the film was based around the characters' obsession with their appearances, it was important that the faces were made of silicone. Plastic, even if painted to be realistic, would always lack the life-like glow and translucency that real skin has. The heads were sculpted and molded, however that mold was used to cast plastic heads first. The back of the plastic head is marked along the “hairline” and is dremelled away, creating a smooth space that was ⅜” deep. Then the hole was refilled in two layers to create two “caps”. These caps would click back into the hole of the head and would later be cast in silicone. The reason why caps were made was because each body needed multiple heads and since hair was such a long, tedious process, a single hairstyle could be punched into one cap and that cap could be moved onto each of the different expressions. This eliminated the need for each head having a hairstyle.
Then a silicone mold is made of the plastic head with the caps cut out. A head core, similar to a skull, holds the eyeballs, eyelids, and helps the silicone grip onto something. Additionally, it lines up the head caps so that they are perfectly registered on every head. The outer silicone color is painted onto the mold and, with the head core inside the mold, injected with the inner silicone color. Details, such as blushing, contour, and makeup, are silicone painted afterwards.