As Karl Malden's police sergeant character leaves his desk, he calls for a colleague named "Sekulovich" to toss him his hat. Malden's birth name is, in fact, Sekulovich.

The original script was written in 1944 and sat on the shelf at Warner Brothers until 1963 when at long last it was finally in production.

This was Jean Hagen's final film before her death on August 29, 1977 at the age of 54.

Bette Davis declined a role in 4 for Texas (1963) to do this film.

For Bette Davis, this film was a reunion not only with Paul Henreid but also George Chandler; they were both contract players at Warners in the 1930s. This was their first picture together since Front Page Woman (1935) 29 years earlier.

Bette Davis smokes 16 cigarettes in the course of the film.

Lana Turner turned the film down because she didn't want to play twins.

Bette Davis originally wanted James Philbrook of The New Loretta Young Show (1962) to play the role that eventually went to Karl Malden.

This was the third time that the director Paul Henreid worked with Bette Davis. He acted in two other films alongside her: Now, Voyager (1942) and Deception (1946).

Tony's car is a 1959 Alfa Romeo Giulietta Spider. Only 2,796 were made. At auction in 2017, in excellent condition, this car could fetch well over $100,000.

Robert Aldrich was a candidate for directing.

When Eadie is dropped off at her bar by George the chauffeur, there is a building under construction in the background. It is the headquarters for the Los Angeles Department of Water and Power at 111 N. Hill St. on Bunker Hill. It was completed in May 1965 and renamed the John Ferraro Building in 2000 after the city's longest-serving city council member.

Margaret's car is a 1955 Bentley S1 with body by coach-builders Freestone & Webb. It was the only one made with left-hand drive, and was owned by Jack Warner himself. As of 2017 this car is part of the Blackhawk Collection in Danville, California. At auction, given its provenance and condition in 2017, it would be expected to fetch well over $150,000.

In Febuary 2019 the Bentley S1 carried just-married opera soprano Linda Watson and husband Peter Ochsenhofer from the church in Orinda to their wedding reception in Lafayette, California.

This film is arguably a "remake" of the 1946 Davis vehicle A Stolen Life, but in that film, Bette merely happens to survive a boating accident in which her twin sister drowns, while in this film she outright murders her wealthy twin in order to take her place.

The pistol Edith uses to kill Margaret is an Iver Johnson Top-Break.