The shot of Clarissa contorted in bed was achieved using Devin DeVasquez and one of DeVasquez's friends, who was buried under the covers with her legs exposed. The two were placed at such an angle that it appears to be Clarissa's body twisted in a strange position.
The original script began with the scene where Billy creeps back into his house after leaving the hospital, with the first hour of the film being told as a flashback. To avoid audience confusion though Brian Yuzna opted instead to have a new opening written that's more of a foreshadowing to later events rather than setup for a flashback.
Billy Warlock was hesitant to show his bare ass during his sex scene, as it was scripted. Luckily the scene was scheduled late in production and he was able to refuse the shot without being fired. He jokingly claimed he doesn't have a nice butt.
The trivia items below may give away important plot points.
The original script had a different finale where the secret society was revealed as a cult that was out to sacrifice Billy for is own crazy reasons. Brian Yuzna, however, wanted a much more fantastic revelation, so he and makeup artist Screaming Mad George came up with the idea of the society being monstrous creatures that ate the lower class.