What Could Have Been: There was originally going to be a subplot involving poison, which the girls' mother drank before hanging herself. Su-mi was to take some of the poison and use it to contaminate Eun-ju's medication. Kim-jee Woon realised that this could have become problematic as that would mean Su-mi was poisoning her own medication, so it was dropped, and Su-mi's own desire to die manifested itself in a different way in the finished product.
Su-mi father initially fiddled with a video camera (only one scene of that is seen in the final product) to reveal to Su-mi her condition.
There were initially scenes of the doctors watching video recordings of Su-mi and putting both a psychological and supernatural diagnosis on her condition. The director realized that naming or explaining the diagnosis would ruin the ambiguous charm of the movie.