Two things you should understand before you see this movie.
1. While it was the highest grossing film in South Korea the year of it's release, this is by no means a reflection of it's ability to appeal to a mainstream western audience, especially a modern American audience. Simply put, this film requires not only a proper attention span, but the mood to really dive into the characters minds.
I've heard many people complain that the movie starts out boring, but that never crossed my mind because i was genuinely charmed by the friendship of the two sisters.
2.This is not a horror movie, it's a psychological ghost story, with a heart, and if you let yourself in, it will make you feel happy then shred your nerves to pieces. There is little blood/gore. The movie slowly builds up suspense and dread and brings you into the protagonists psyche by never revealing too much.
The best thing about this movie is that it delivers an equally beautiful opening and ending sequence. And while ultimately answering your questions, leaves you with the same uncertainties you had going into the film.
Do not expect a 'thrill ride' or a lot of jump scares. This movie is atmospheric, beautiful, complicated and despite what others may say against this statement, i truly believe this is a masterpiece of modern cinema.
My advice: Watch it alone, be attentive, turn off the lights and let yourself be immersed.