The ultimate problem with any movie that relies on some sort of "twist" is that you can only be surprised once. If the movie is fairly well made..."Us", for example...or a couple of other famous titles featuring a few famous names that I won't even bother mentioning here, because both of those films rely on the exact same gimmick as this one (if you watch this until the end, you'll figure out which ones I'm talking about)...they bear repeat viewing. Not just to figure out how you were fooled in the first place, but you get to absorb the character relationships from an entirely different perspective. However, if you're able to figure out what's going on before the movie pulls back the curtain, then all you're left with is the story and the characters, which bear closer scrutiny. In the case of "Goodnight Mommy", it would have probably helped if the entire film weren't told through the eyes of two twin boys with no personalities or emotions between them. The initial premise is flawed from the get-go: the boys believe that their mother is somehow acting "different" post-surgery (she spends the first half of the movie with her entire face bandaged), but we're NEVER given a scene with her interacting with her sons PRE-surgery, so we have no idea what their relationship was like before. So we sit and watch these two dullards stare at each other, stare out of windows, stare at their mother (who may not even be their mother), play with (and stare at) cockroaches, they even run away from home where they stare at a few townsfolk. Sometimes, they back-sass their mother (who may be a stranger) and we have a few scenes where they devise some vague plan to find out the truth. This is the first 60 or so minutes of the movie. If you haven't checked out by then (because you'll figure out what's going on within the first 5-10 minutes), the second half picks up steam by becoming a foreign "art film" version of "Saw", which admittedly for this viewer had a few tense, but still frustrating moments. Then, after all the buildup, we finally get the punchline, which, again, even Stevie Wonder could see coming from 100 miles away...and then, it's over. And we're left with no insight on what led up to this and why, just a "GOTCHA!" moment, which honestly, has been done much better in movies far below the "prestige" of movies like this. In fact, the idea that this film thinks it's smarter than it actually IS gives it an air of pretentiousness that really leaves a bad taste in your mouth hours after you've seen it. In spite of it all, it DOES have a few good things going for it. The Mother/Stranger (Susanne Wuest, "A Cure For Wellness") is quite good, as is the cinematography. As for the twins, played by Elias and Lukas Schwarz (whose play two characters named Elias and Lukas), maybe if they were given a better script and better direction other than "blank stare", I would have cared more about them and their predicament.
Final Verdict: there's better ways to waste two hours.