Twin children are one of my least favorite horror tropes - it's all been downhill since "Come play with us, Danny." Usually now it's something lazy like James Wan putting them in dolly-dresses and making the camera twitch around them - as a visual, they're played out. So how does Goodnight Mommy make them work so spectacularly again? Well for one the film has very little interest in filling in the gaps in what we know about what's happening - there's mention of an accident, there's Mommy with her bandaged face, there's the isolated house in the middle of nowhere with nobody around... but the movie never gives us anything to hold onto for too long, up to and including keeping those damned twins straight. It's shuffling and reshuffling its hand every chance it gets.
This isn't a complaint I'm making - I felt totally off-balance but never not intrigued by what was happening, right up through the tense-as-fuck final act. Hell when big revelations start coming they turn out not to be -- they only deepen the movie's mysteries; faces under faces. Like Mommy's bandages coming off... you think that's gonna help you out? Goodnight Mommy stares you dead right in the eyes and marvels with its steady inscrutability. You stare and stare at it and its like a face in a dream, where all its features are all the features at once. It's an infinite matryoshka, nesting something eight-legged and triple-faced and maybe mighty high insane.