I originally posted this on my thread regarding Dan, but I think it provides a huge clue as to what is going on in this film. So, I'm reposting it in this new thread.
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Dan says: "There's a man in back of this place. He's the one who's doing it. I can see him through the wall. I can see his face. I hope that I never see that face ever outside of the dream."
Remember that Club Silencio teaches us about illusions. What is an illusion device that we use every single day?
Ready for the answer?
A mirror.
What is this wall he can see through? The pane of glass that makes up the mirror.
Where's the 'back of this place'? The other side of the mirror. The reflection.
What is this man doing? Looking back at him.
Who does Dan see? Who's this man? Himself.
Betty: "You look like someone else."
Rita: "Oh, I was just looking at myself again."
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I think this may be a major, major clue.
If a person has serious mental issues it could be very possible for them to see a photograph of someone (an actress' photo resume, perhaps?) and mistakenly assume that they are seeing their own reflection in a mirror.
When Rita comes out of the shower and stands in front of the mirror what does she do? She assumes the identity of the 'reflection'.

