- Actor Allusion: At one point Adelle strikes Helen with a crumpled up letter, mimicking a similar scene from Lolita where Charlotte throws Humbert's diary at him. Shelley Winters played Charlotte.
- The Cast Show Off: Debbie Reynolds shows off her dance skills in several numbers throughout the film.
- Doing It for the Art: Debbie Reynolds produced the film uncredited and put up $800,000 of her own because she loved Henry Farrell's story outline.
- Dyeing for Your Art: Yvette Vickers who plays Mrs Barker, was asked to dye her blonde hair red so as not to be confused with Debbie Reynolds.
- Executive Meddling: The censors objected to several things, such as a kiss between Helen and Adelle. Likewise Helen's murder of Adelle was longer and more graphic, but had to be cut down to avoid an R-rating.
- Follow the Leader: The film was put into production to capitalise on the success of What Ever Happened to Baby Jane?. Filmmakers also wanted the film to be "the next Psycho".
- Reality Subtext: Shelley Winters apparently got Lost in Character throughout the shoot, driving a lot of the cast and crew insane in the process. This eerily paralleled Helen's deteriorating sanity.
- Throw It In!: Shelley Winters ad-libbed Helen kissing Adelle after they dump the body in the sewer. Censors however objected to it and it didn't make it into the film.
- What Could Have Been:
- Rita Hayworth was the first choice for Adelle but she had just been diagnosed with Alzheimer's disease and couldn't take it.
- Estelle Parsons was up for the role of Helen before it eventually went to Shelley Winters.
- Working Title: The Best of Friends, which had to be changed to avoid confusion with the film Such Good Friends.
Trivia / What's the Matter with Helen?