It's uncanny how realistic Östlund's films feel.

As you say, the dialogue, the small details like the janitor on the hotel and especially the way it's filmed. The long, static wide shots are perfect for making you feel uneasy. At times it's almost like you're trapped with real person. You want to look away, but you can't.
De ofrivilliga is even more cringeworthy, but
Force Majeure a more complete film.
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Oh, and I didn't know you watched the
Before films.