Shizuko Yamamura is based on a real person, Chizuko Mifune, who was born in 1886 in Kumamoto Prefecture and was rumored to have the gift of foresight. After a demonstration in 1910, she was proclaimed a charlatan and committed suicide a year later by ingesting poison.
The effect of Sadako coming out of the well was accomplished with only one simple special effect. Rie Ino'o, who is a student of the Kabuki theatre, which uses exaggerated motion and jerking movements to emphasize emotion, was heavily involved in the development of the Sadako character. Ino'o was filmed walking backwards and the film was run in reverse - the end result is Sadako walking forwards with unnatural motions.
The 2001 UK DVD release back cover carries a disclaimer, stating that the distributor accepts no responsibility for "for any injuries or fatalities that may occur during or after the viewing of this videocassette."
The characters of Sadako and Shizuko are named after and loosely based on two real women. Both were taken under the wing of Professor Fukurai Tomokichi and, whilst not actually mother and daughter, Takahashi Sadako did work with the professor soon after Chizuko's suicide. Neither woman possessed the gift of nensha, but another student of the professor, Nagao Ikuko, was believed to have this power.
The American trailer for Ringu pronounces the word as "ring-oo," but in fact, Japanese words must end either with a vowel or an -n. The "u" should be pronounced as a softly vocalized schwa; the word is taken from the Anglo-American verb, regarding the cyclical nature of Sadako's curse, and has nothing to do with the visual circular rings of the American remake.
In his film script, Hiroshi Takahashi changed the protagonist's gender (from male to female), name (from Kazuyuki Asakawa to Reiko Asakawa), marital status (from married to divorced) and child's gender and name (from daughter Yoko to son Yoichi).
On its 1999 Hong Kong release, Ringu earned HK$31.2 million (US$4.03 million) during its two-month theatrical run making it Hong Kong's highest-grossing Japanese-language film. This record was later beaten by Stand by Me Doraemon (2014) in 2015.