The viewer is supposed to feel some sympathy for Justine, and we are privy to her thoughts via occasional voice-overs. She sees herself as trapped in an excruciatingly boring and pointless life.
I think the movie over-all works, so I've left a lot of developments unspoiled here and I recommend it.
Reilly excels as an actor (he always does, and in this movie he is doing the "Cellophane Man" from Chicago in a very different time and place).
Aniston also excels here.
I can't say the same for Gyllenhaal, but he does what is required of him.