The first nighttime scene in Su-yeon's bedroom. Her door slowly creaks open of its own accord... she shrinks back in fear... and after a moment of tenseness, a hand suddenly grips around the edge of the door. Gaaaah.
The scene where the wife of the girls' uncle starts having a fit. It is frightening enough the first time around, but, on a re-watch, if one believes the theory that she was being possessed, the scene takes on a very different light.
The morning after Su-mi is told that Su-yeon is dead. Moo-hyeon leaves the house on an errand, and Su-mi is left in the house, becoming more and more terrified as her delusions spiral out of control.
Oh GOD this movie. To be fair, it falls under "it's a horror film, freaking you out is the point" but the particular way they do so... it's just... mean as hell. They first proceed to do some fairly tame Jump Scare tricks and nightmare sequences, but it's par for the course. It is a horror film after all. Of course, it only fools you into thinking it's gonna be this tame the whole time. The cutoff point is the "dinner freak out" scene. From that point on, the entire rest of the movie suddenly takes a huge ramp in the scare factor, putting you on a psychological ride through hell. The scariest parts were the ones where nothing happened, and yet, and yet... daaaaaamn. The movie is also a Mind Screw of epic proportions, so if you didn't understand it, you'll have to watch it all over again — if you can.