- Acting for Two: Pat Healy plays two characters in the movie: Sir Edmund William Godfrey, who is murdered by the three thieves in the first scene, and the young pharmacist Julianne Moore yells at.
- Big Name Fan: Ingmar Bergman was a fan of the film.
- Cast the Runner-Up: According to Thomas Jane, he was supposed to play two different characters in the film. But the scheduling overlapped with Under Suspicion so he was only available for the brief cameo as the young Jimmy Gator.
- Creator Backlash: Paul Thomas Anderson admitted that he finds the film far too long now, and would have cut some of the story lines from the finished product. He never specified what stories he would have cut. In an interview with Marc Maron in January of 2015, he was asked if he had the opportunity to re-cut the film. He replied, "I'd slice that thing down. It's way too fucking long. It's unmerciful how long it is." He added that "maybe a few" trajectories in the film's plot lines could've been eliminated.
- The Danza: Luis Guzman has a role as a contestant on "What Do Kids Know?" named Luis Guzman.
- Deleted Role: Orlando Jones' role, as "The Worm"—the father of the rapping boy and also the murderer of the man in the closet—was left on the cutting room floor. This makes Dead Guy in Closet's Murder an Unreveal and makes the boy's presence in the film more or less random. In the final cut Jones appears only as the mysterious hooded pedestrian that Officer Jim chases before losing his gun.
- Mid-Development Genre Shift: This was originally supposed to be a short and intimate film. But the more Paul Thomas Anderson worked on the characters the more intricate it became. So the plot expanded from there.
- Name's the Same: The gay barfly (played by Henry Gibson) is named "Thurston Howell".
- Playing Against Type: John C. Reilly was tiring of always playing heavies so he asked Paul Thomas Anderson to write him a part in which he falls in love with a girl.
- Production Posse: Many of the film's major and minor actors return from Paul Thomas Anderson's previous film, Boogie Nights; the only exceptions among the former are Tom Cruise, Jason Robards, and Jeremy Blackman.
- Real-Life Relative: The WWDK Show Director's Assistant is played by William Mapother, Tom Cruise's real-life cousin.
- Throw It In!: According to Philip Seymour Hoffman, during the deathbed scene, everything after Frank's 'I'm not going to cry for you' was improvised by Tom Cruise. Cruise didn't feel the scripted lines worked, and Paul Thomas Anderson told Cruise to think of when his own father died and to let it move him. During the next take Cruise broke down sobbing, resulting in the scene seen in the film. Hoffman stated Phil's reaction to Frank sobbing was his own, since he didn't know Cruise would enter such a zone and he felt the purity of Cruise's emotion.
- Uncredited Role: According to her official site, Miriam Margolyes asked not to be credited in this film due to the size of her role.
- What Could Have Been:
- George C. Scott was offered the role of Earl Partridge, but turned it down because the script was, in his words, "..the worst fucking thing I've ever read. The language is terrible." Marlon Brando was also considered for the role.
- Paul Thomas Anderson wanted to cast Burt Reynolds for an unspecified role. But Reynolds had become angry with Anderson during the promotional tour for Boogie Nights and turned the role down.
- Debra Winger was considered for the role of Linda Partridge.
- Along with the scenes further explaining The Worm, there were many other scenes and dialogue exchanges included in the script but not used for the final film, including:
- A scene in which Jimmy talks with Paula, the dancer he is having sex with in the opening sequence.
- Extended dialogue between Gwenovier and Frank, about "Subjective human experience and terrible things", which was referenced in the final cut but not shown.
- A brief dialogue between Jim and Claudia after he asks her on a date, in which they both find they have the same favorite restaurant.
- We see what happens to Stanley between the events of that day's game show, and him talking to his sleeping father.
- Further explanation during Earl's monologue concerning the reasons he mistreated his first wife.
- The tone of Frank's confrontation with his father is considerably different, with him showing less hostility, more genuine sympathy and concern, and attempting to wake him up.
- It is made clear in the script that Jimmy doesn't survive the fire in his house caused by the destroyed television.
- We see Marcie's confession for what happened to the "guy in the closet".
- Write What You Know: According to Philip Baker Hall, the scene where Frank T.J. Mackey visits Earl Partridge on his deathbed is loosely based on Paul Thomas Anderson's experiences of watching his father, the late WABC-TV announcer Ernie Anderson, die of cancer.
Trivia / Magnolia