Actor Allusion: In the musical— Glenn Close portrays a lonely troubled woman slowly driven to insanity due to her unhealthy obsession with a man. Eventually she's pushed over the edge and goes on a murderous rampage... But enough about Alex Forrest in Fatal Attraction.
Executive Meddling: Probably for the better. The original cut of the film involved Joe Gillis's corpse talking to other corpses in the morgue, which made test audiences laugh uproariously. All that survives of this early opening consists of script pages and some silent footage of morticians transporting and storing the corpse.
Mean Character, Nice Actor: Gloria Swanson was worlds more stable and down-to-earth than Norma Desmond. Very notably, Swanson handled her change of fortune much better than Desmond: When it became apparent that her film career was over (at least in any roles besides a White-Dwarf Starlet) she focused more on her theatre and television prospects, which, while nothing earth-shattering, were fairly successful and kept her in the public eye.
Reality Subtext: Norma is played by Gloria Swanson, who was a silent film star, and worked with Cecil B. DeMille. Max is played by Erich von Stroheim, who was a silent film director.
The movie that Norma and Joe watch together was comprised of footage of from Troubled ProductionQueen Kelly (1929) - directed by none other than Erich von Stroheim.
And Cecil B DeMille referred to Swanson as "young fella" when both were getting started in Hollywood.
The Waxworks, of course, were real former Hollywood stars then considered has-beens, including Buster Keaton.
Those Two Actors: After the success of the film, this was attempted with William Holden and Nancy Olson; they appeared in three further films together, but none of them was really successful.
Type Casting: Filmmakers just rehashed Norma whenever they wanted to give Gloria a part in a film after that, and some people still confused Swanson with Desmond.
Several other old-time actresses were considered for the role of Norma Desmond, including Pola Negri, Mary Pickford, and even Mae West, who was turned down after one meeting with the producers, who said they realized that she was still both too dignified and too pretty to believably sell a has-been character like Norma. Montgomery Clift was originally cast as Joe Gillis, but pulled out - reputedly because he himself was dating a much older (and arguably washed up) singer at the time.
Stephen Sondheim began work on a musical version, before the release of Andrew Lloyd Webber's play. He abandoned the project due to Billy Wilder saying that he'd prefer a Sunset Boulevard opera to a musical.