Whatever your "in-" is, we are about to find it.

Contrary to legend, the prop department did not use a real severed head. They used a silicone replica with a hinge in the jaw. The "blood" was a mixture of corn syrup and red dye. But the ? The box had to feel real. Brad Pitt’s horrified reaction is authentic because Fincher changed the reveal at the last second. Pitt did not know what the prop would look like until the camera was rolling.