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.
As with the original Ju-on series, Toshio has been portrayed by three different actors. In the first film and in flashbacks/photographs for the second and third films, he was portrayed by Yuya Ozeki (who also portrayed the version of the character in the two theatrical Ju-on films). In the second film, he was portrayed by Ohga Tanaka. In the third film, he is portrayed by Shimba Tsuchiya. (Unfortunately, the difference is rather jarring in the third movie, as Toshio looks a lot older than in the previous films.)
Not only that, but Takako Fuji, who has played Kayako from the very beginning of the Ju-on saga up until The Grudge 2, has been replaced in The Grudge 3 by Aiko Horichi (although Fuji does appear as Kayako in the third movie's flashbacks and photographs). According to Takako Fuji herself, she was getting tired of playing the same role over and over again. Also, in the Tales of the Grudge shorts, Kayako is portrayed by Anna Moon.