Executive Meddling: Kayako did not have much of a backstory in the original film series, and Takashi Shimizu wanted to keep it that way. However, he was told to give her a backstory for The Grudge 2, which he reluctantly went ahead with . In an interview, Shimizu also stated that executives kept insisting that Kayako should have "a twin", which baffled him. In The Grudge 3, Kayako is revealed to have a sister (though not a twin sister), which has to make one wonder if that was a case of executive meddling, too.
This also isn't the first time that Sarah Michelle Gellar and Jason Behr (Doug) have appeared on-screen together - Behr played Billy Fordham in the Buffy the Vampire Slayer Season 2 episode "Lie To Me". Additionally, Clea DuVall (Jennifer) appeared in a Season 1 episode, "Out of Mind, Out of Sight".
Additionally, Clea DuVall and KaDee Strickland (Susan) have previously appeared in another movie together, Girl, Interrupted.