On the DVD commentary Sarah Michelle Gellar remembers that Jason Behr had previously guest-starred on her show 'Buffy the Vampire Slayer' but not Clea Duvall who also appeared in one story 'Out of Sight, Out of Mind' as a girl who turns invisible. This is because Gellar never had any scenes with Duvall's character whilst she was visible so the actresses never actually met.
According to the DVD Commentaries and Featurettes, the house in The Grudge (2004) was built completely on a sound stage. It was based almost entirely on the house in Ju-on: The Grudge (2002). It was built like a normal house, the 'house set' had two floors so that cuts did not have to be made when a character was walking upstairs. However, the attic was built separately from the house to allow for more room when filming.
According to the DVD commentary by the cast and crew, the first of appearance of the cat right when Karen picks up Kayako's journal was not in fact a real cat, but a fake cat held by a crew member, racing it in front of the camera.
The trivia item below may give away important plot points.
There is a deleted scene after Toshio attacks Matthew, where Matthew becomes possessed by Takeo's ghost and kicks a visiting Susan out of the house. Takashi Shimizu cut it because it took focus off Kayako.