u/Fn4cK

My theory is as the title says. The story HAS been written, you can make your own choices, but there are fixed outcomes and scenarios for all decisions you make. For whatever reason, the souls of Tabitha and Jade have been caught in finding the solution out of perdition, or whatever the hell the dimension is they're cought in.

Julie's bookmarks are the pages you are told to flip to after certain decisions. There are a finite number of outcomes, and she is therefore able to visit them without her actions affecting the story.