From Redditor u/DaveInLondon89:
In Se7en , Fincher never told John C. McGinley that the corpse used for Sloth was actually a living actor in heavy makeup. McGinley's character's initial shock when Sloth wakes in the movie was, in fact, McGinley's genuine surprise from the first take.
According to actor Michael Reid MacKay, the makeup was a process:
It started with airbrushing tiny little veins on my body, and then there was a step where they did some more painting, and then they put the gelatin sores all over me...
MacKay also stated that the crew members hid him away so the actors didn't see him.