According to the director, the actors studied real rituals from shamans in order to accurately portray them in this film.
A leading actress Kim Go-eun, who starred as Hwarim, a shaman, said on the interview that she was bit worried about being obsessed or other supernatural things she might experience. This was because everything she did on the scene (praying, exorcism for example) was exactly same as real shamans do. But a real shaman who assisted production of film talked to her that she doesn't need to concern such thing. And one more thing, director is christian himself, serving as a deacon for a local church.