I just wanted to know when everyone realised what was actually happening. Maybe watching A Beautiful Mind recently helped, but I thought something was off about Pitt and Norton about midway through the film.
Here are some things I picked up:
The way no one interacted with both of them at the same time, and especially how it was always one of them doing something in the basement in the 'fight club' whilst the other was just spectating.
When Norton opens up the door to a naked Brad Pitt, and Marla asks who it was when she obviously would've known who he was. Also when, at their house, Marla and Norton are talking whilst Brad Pitt is in the basement talking to Norton; something felt off.
I thought for a bit that maybe Marla was the imaginary one (the way she just turns up at those rehabilitation/support groups)
When Norton punches himself in the office in front of his boss, I realised he could've been doing that to himself the whole time, and that Pitt's character wasn't real.
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Anyways, what a great film with some fantastic acting and hard-hitting themes. I'll probably have to watch it again to fully understand and appreciate it though.