Originally it was much worse and New Line Cinema had to step in and order Sean Cunningham to put Jason in the movie. Cunningham, producing the movie himself and not New Line, was making a movie called "Friday the 13th: Heart of Darkness" which revolved about Jason's evil brother Elias digging up Jason's corpse and eating his heart, and then going on a body hopping rampage around Crystal Lake. Why? Because at the time, Cunningham hated Jason, whom he felt was the creation of Steve Miner.
When Cunningham sold the project to New Line, he'd just told them "I want to make a 'Friday the 13th'," and New Line presumed like anybody would, that meant a Jason movie. They had finished principal production with only about 60 minutes of useable footage when New Line caught wind. They told Cunningham that he was to put Jason into the movie or they wouldn't release it. His back against the wall, Cunningham called in a writer working for him named Dean Lorey who had a weekend to fit Jason into the storyline they already had. Jason, Creighton Duke, "Elias" becoming Jason's father, and the coroner characters are his invention. Kane Hodder was hired to play Jason and they set about the reshoots, turning it into a "proper" Friday.
The problem with this is that NOBODY involved will cop to the above story, chiefly because it makes them look bad. Dean Lorey started to talk about it in the audio commentary for the DVD, but he either gets interrupted by Adam Marcus or stops himself and they never go back to it. Apparently, effects man Howard Berger never got the memo to hush up, though, because in the book "Crystal Lake Memories", he mentions the "Heart of Darkness" script they began work on and how it began with a scene where Jason and his mother copulated to produce the brother character "Elias."
This story needs to get out because people need to understand that it's a wonder this movie makes any damned sense at all with its ungodly troubled production with a producer who had no business making it.
