Emma Watson previously starred with David Thewlis in the Harry Potter franchise.
Emma Watson insisted on not being in the rape scene at all, even for simple close-ups on her face. She wanted people to be able to tell without any hesitation that it was a body double and not her that was naked. This is why the hooded girl's face was digitally blurred and her naked body was only shot from behind.
When Ethan Hawke dozes off in his recliner, right before the nightmare sequence, when the black mass takes place, there is a poker by the fireplace, but when he awakens, the poker is gone. This insinuates that in fact, it was not a dream, but that the mass did in fact take place. However, the entire point of the movie, is that it had to have been.