When Dwight "Bucky" Bleichert goes searching through some photographs, you can see a real autopsy photo of Elizabeth Short.
Mia Kirshner was originally intended to feed lines to potential actors in screen tests, but 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.
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.
David Fincher originally planned to direct and make a five hour version for television. He subsequently left the project, apparently because he doubted that he would be able to make the film exactly the way he envisioned.
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 conclusively identified or apprehended.