While in production, cowriter/director Tom Holland was stricken with Bell's Palsy, a virus that paralyzed one side of his face. The effects could have been minimized had he gotten a steroid shot immediately, but the producers insisted he keep working, so it was 36 hours before he got to a doctor. It took more than a year and a half for him to fully recover.
Originally the crew planned to do a more gruesome FX makeup which would have had Billy Halleck's flesh dangling off of his protruding jaw and cheekbones. Partway into filming they decided that this look was too horrific.
Depending upon the stage of his character's deterioration, Robert John Burke had to spend four to six hours each day in the makeup chair.
At his thinnest, Billy Halleck weighed 120 pounds which was a challenge for the FX crew, as actor Robert John Burke weighed 160 pounds at the time of production.