Which particular scene in the movie was the most frightening for you?
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That expression or lack there of when He opens up the letter with that dramatic pan out. It gives a sense of unexplainable dread and doom as we don't know what's in the letter and what he's thinking, it's left for us to fill in the gap which kinda taps into the depths of subconscious.
The fox scene comes second for me. When we have been watching the first half anticipating for something to finally happen, then suddenly ''chaos reigns'' it's like the movie talks to you.
It works well IMO in the context of the schism between the couple, the mood of isolation where us the rational viewer are put in the shoes of He who's whole role of the rational one is being challenged.
The super explicit violent scenes are more of a cringe than terror. Genital mutilation fits very well with the theme of the movie which deals with the interplay of the genders.