I believe the best way to describe this movie is an unpleasant experience. Unpleasant in the sense that it leaves you in a constant feeling of dread and uneasiness. In my opinion, I feel the setting of the movie really played the biggest part in that aspect. Fear is the lack of knowledge of the unknown. This movie really plays with the idea of the known knowns and unknown unknowns, in the sense that as the movie progresses the known knowns become the unknown unknowns, which made that feeling of uneasiness even much more existent. This feeling was more apparent in the final act of the movie, which honestly felt like a living nightmare. I truly have never been so frightened by a movie before, and it was more of an uncomfortable “What in the world is happening” kind of fear. It seemed like the writers really understood what fear really is and what kind of situations that can induce those feelings, and that's something I can appreciate.
In terms of characters they weren’t bad, I don’t really have any strong opinions about them. But, what I would say is that they were well performed. The musical scores were really enjoyable, It really gave off an eerie and dreamlike vibe that the movie is all about. In the end, it was like watching a nightmare dream you had where you went on some mission to save the world but you wake up due to some frightening or unpleasant scene you saw. It gets a 4/5 in my books.
PS. I suggest you watch this movie in the dark, and on your own for maximum enjoyment.