When we see Kayako crawling down the staircase towards the end in the infamous staircase sequence, she is contorting and bending her body and limbs in ways that seem humanly impossible. Takako Fuji, being a trained contortionist, performed the stunts herself; the effect is not a result of a trick shot or digital manipulation.
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.
In the original film of which this is a remake of featured a subplot involving three schoolgirls encountering the curse. Although it was omitted from the remake it was restored in the film's sequel The Grudge 2 (2006).
Unlike the other J-horror remakes like The Ring (2002), Dark Water (2005), Pulse (2006), and One Missed Call (2008), this is the only film out of all the others that actually remains in Japan instead of moving the story to America.
Due to American violence ratings and the commitment to a PG-13 rating, the off screen implication that Takeo stabbed Kayako with a box cutter was omitted from the film (it was present in the original film).
The trivia items 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 despite being present in the original film.
In the original film, Rika (Megumi Okina) is actually killed by Takeo Saeki at the film's conclusion. Rika's American incarnation Karen instead manages to survive (though she later dies in the sequel). A remixed version of the original ending is re-introduced for The Grudge 2 (2006) involving Aubrey, Karen's sister.