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.
The original ending was going to have Su-mitake a fatal dose of medication. After Su-yeon's ghost briefly visits Su-mi in her hospital room, finally getting her to respond, she returns to her former room to (presumably) kill Eun-ju in vengeance. Afterwards, Su-mi lies back in her bed with a small smile on her face. The alternate ending continues, showing a discarded and nearly empty pill bottle on the floor, and a brief shot of a lifeless Su-mi, who committed suicide, presumably to finally reunite with her mother and sister.
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.