1) It doesn't hurt physically at all, but the more a character mentally struggles against it, the more mental pain they'll feel! So those who fight against the influence could end up with something akin to a migraine for their attempts!

2) They could see what the MMM does to the ones before them, so they might have a general idea. It's up to the players if they want their characters to be told or not -- there are enough witches to handle them and not all of them will treat the prisoners the same way