The screen test that characters keep watching are not in the James Ellroy novel. They were a plot device created by screenwriter Josh Friedman. The real Elizabeth Short claimed to have screen tested at major studios but no such footage has ever been found.
James Ellroy's book was based in part on the true story of the murder of Elizabeth Short, an aspiring actress who'd moved to Hollywood in the mid-1940s from Medford, Massachusetts. A drifter and hanger-on who never managed to break into films, she disappeared in early January 1947. On January 15, 1947, her horribly mutilated corpse was found in an empty lot in south-central Los Angeles. Her murderer was never publicly identified or apprehended.
David Fincher had originally planned to direct and intended to make a three-hour version shot entirely in black and white. Fincher subsequently left the project, apparently because he doubted that he would be able to make the film exactly the way he envisioned.
Mia Kirshner was originally intended to be feeding lines to potential actors in screen tests. However, her performance so caught the attention of director Brian De Palma and writer Josh Friedman that she was cast as Elizabeth Short and her role expanded significantly compared to the novel.
Brian De Palma's initial cut ran at roughly three hours and was a faithful adaptation of the book, with more time dedicated to Bucky's psychological breakdown during the investigation and his obsession with avenging the Dahlia. James Ellroy was shown a print of this version and wrote an essay praising it; entitled "The Hillikers," it was published in re-issued prints of the novel, which were released before the film premiered. In the interim between Ellroy's having seen the director's cut and the publication of his essay, the film was significantly edited. After seeing the theatrical cut, Ellroy refused to comment on it, except to tell the "Seattle Post-Intelligencer", "Look, you're not going to get me to say anything negative about the movie, so you might as well give up."
The character of Lorna Mertz, played by Jemima Rooper, was based on a figure in the real case named Norma Lee Myer. Under the name Lynn Martin, she had been living with Elizabeth Short and another girl named Marjorie Graham in a hotel shortly before the murder. At the time of the investigation, Martin was 15 years old and had several arrests on her record.
Mark Wahlberg had initially signed on to play Lee Blanchard opposite Josh Hartnett, but scheduling conflicts with the planned The Brazilian Job (????) prevented him from taking part even though production on The Italian Job (2003) sequel was eventually pushed back.
Cate Blanchett has an uncredited and unmentioned cameo in the film. In the scene where Josh Hartnett's character is in a lesbian bar asking questions and he sees Hilary Swank's character for the first time, Cate Blanchett is one of the dancers while the song is being sung and there are at least two shots of her when Josh Hartnett's character is asking some women at a table if they had seen Elizabeth Short.
Mia Kirshner had actually read James Ellroy's novel of the same name back in the 1990's as she had an interest in the actual murder case of The Black Dahlia. She was always told that she bore a resemblance to Elizabeth Short and should audition for the role should there ever be a film based on the case, which is exactly what happened.
Both Aaron Eckhart and Scarlett Johansson have appeared in superhero film adaptations with Eckhart appearing as Harvey Dent / Two-Face in The Dark Knight (2008) and Johansson appearing as Natasha Romanoff / Black Widow in the Marvel Cinematic Universe.
Marjorie Graham, a former roommate of Elizabeth Short (see above), was originally a character in the screenplay. She was written into one scene, occurring right after Sherryl Saddon's scene, in which she insinuates that Elizabeth Short and Lorna Mertz were consorting with a lesbian. This scene was written out, and Sherryl performs this function in the final film.
Despite the title of the film, the subject matter, the source material, and the real life crime that inspired the film, Elizabeth Short (The Black Dahlia) and her murder do not become relevant to the plot until 25 minutes into the film.
Despite top billing as part of the cast, Hillary Swank does not appear as her character until 45 minutes into the film, and throughout the rest of the film she only maintains about 15 minutes of screentime.