Here's Part 2 of "The Creek" and naturally after seeing the whole thing there will be lots of questions about the ending. I will address them here but be warned if you
haven't seen the whole thing as there will be SPOILERS:

*SPOILERS*

Yes the entire "slasher film" was all in Joanna's head. When she was a child she was camping with her father and he was murdered. She was forced to wait in the cellar with his body for three days before the police arrived. As a result she spent two years in an asylum and the rest of the years after that on medication. Three days before the trip she is taken off the pills to test her progress. The story Jack made up about the killing spree awakened her own suppressed emotions which weren't helped by withdrawals from her pills (as well as her own vivid imagination). She passes out on the beach a few minutes after the others leave to go back to the house.

Just to clear things up, the hallucination starts at the beginning of Part 2. The conversation between Jack, AJ and Carrie definitely happens but everything after that is all Joanna's imagination. There are several clues that hint something isn't quite right with what's going on:
* Inconsistencies with the characters. First of all, Kate seems to enjoy the trip and gets on alright with everyone (and is nice enough judging by her concern at the end). But Joanna does not know her, only having heard about her from Kevin who doesn't have a high opinion of her. Therefore after the beach, Kate is suddenly strangely bitchy and picks a fight with Jack for no reason at all. Secondly Joe and Adrienne aren't the typical slasher movie couple since you see them goofing off and playing cards in the house while everyone else is out. It isn't until the hallucination starts that they actually decide to have sex. Thirdly, Kevin is not a pervert but yet he decides to walk in on Kate in the shower.
* Locations. The location of the storage shed suddenly changes to being under the cabin in the final act. Joanna has imagined that she is back in the cabin where her father was murdered and that is where the cellar was. Also, the bathroom. Do any characters mention a bathroom/outhouse in the area before the hallucination? Jack and Kate's room has a bathroom in it and the dormitory has a bath as well, so why should Kate travel all the way across the woods to have a shower?
* The identity of the killer. It probably wasn't lost on a few people that sometimes there was clearly a woman under the costume and yet others there was a man there. The idea here being that this is Joanna's "first draft". She hasn't decided who the killer will be yet. It may be Jack, as a sort of punishment for him telling that story and organising the trip - he gets possessed by the made-up entity and kills everyone. Or it may be Joanna herself, the story and the withdrawals making her snap and go crazy on her friends. Another possibility is that they are both the killers and the final showdown is a power struggle. Or Joanna just decides at the last minute
to make herself the Final Girl and heroine of the story. There's also no explanation given as to why either of them is the killer. Joanna has still got to work out a motive for her slasher.
* The magic bodies. Whichever of them is the killer they are somehow able to quickly move around all those bodies from the storm shelter and place them around the area so that they will be found at the right times. How s/he does that in the middle of a storm is another matter.
* The storm. The forecast is for good weather all weekend and the weather for the trip is fine until it gets to night. It doesn't start raining until the hallucination starts and doesn't stop until the killer is dead.

The final showdown between Jack and Joanna also is an insight into Joanna's psyche. She has reimagined herself to be in the cellar where she had to hide with her father and she has made Jack the killer. The person who killed her father was never brought to justice so she will never discover his identity. Instead she makes Jack the killer as he was the one who told the story that shook her up. She kills Jack and burns the cellar and the cabin to the ground as a way of separating herself from those memories and casting away her traumatic experience. In a way the "slasher film" she created has helped her move on from them though the figure that attacks her at the end is not Jack but the real person that killed her father, which may suggest she'll never be completely rid of those bad memories.

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