Author Existence Failure: Larsson died of a heart attack shortly after submitting the manuscripts for the three novels. Fortunately, the third book at least resolves most major active plot threads fairly well, so readers aren't left with a gigantic, unresolved cliffhanger as the ending. A fourth book was partially written and some ideas for a fifth and sixth book are recorded. It has been rumored that Larsson planned to write 10 books with these characters. Major plot points left unresolved include the fate of Lisbeth's sister, which was intended to be the subject of the fourth book. However due to ongoing conflicts between Larsson's girlfriend/partner Eva Gabrielsson (who did not inherit the rights to the books because they were not formally married) and Larsson's family...
B-Team Sequel: The Girl in the Spider Web does not use any of the materials Larsson left behind, as those are in the possession of Gabrielsson who does not approve of a Ghost Writer continuing the series.
Cowboy BeBop at His Computer: Not applicable to the author himself, but an article published in an Australian magazine labelled Larsson a Neo-Nazi. This is a classic research flub, as Larsson was a firm Marxist and was well-known for his work against far-right extremist and racist organizations in Sweden. Which included Neo-Nazis (who, in typical Neo-Nazi fashion, regularly sent him death threats). And of course, one of the series primary protagonists is the (Jewish) Inspector Bublanski.
What Could Have Been: Larsson wrote in his spare time as a way to relax, and only decided to try and get the books published after finishing the final draft of Hornet's Nest; he then promptly dropped dead of a heart attack. His girlfriend Eva Gabrielsson is in possession of Larsson's computer, which has at least three-fourths of a fourth novel and is rumored to have detailed synopses on the fifth and sixth books as well, though what may come of this is anyone's guess.
Write What You Know: When he was 15, Larsson witnessed some other boys raping a girl. In Real Life, he was too scared to try and help her, and not only did she become the inspiration for Lisbeth (it was the girl's name), it's pretty easy to map a correlation between his unresolved guilt and the harsh treatment suffered by rapists in his novels. Though this was allegedly Larsson stealing someone else's story
Creator In-Joke: The Girl Who Played With Fire has a scene where EXPO - the magazine Stieg Larsson founded in the 90s - is on display. In the same film, a scene at the police station also has an offscreen voiceover saying "there's a Stieg Larsson here to see you".
Dyeing for Your Art: Noomi Rapace prepared for seven months, taking kick boxing lessons, went on a strict diet and had her eyebrows and nose pierced. She also got her motorcycle licence.
Playing Against Type: Peter Haber, cast as monstrous serial killer and rapist Martin Vanger, is known mainly for two roles. One being a By-the-Book Cop named Martin Beck in twenty-something movies. The other, a role practically everybody born in Sweden during the 80's and early 90's identify him with, is the sweet bumbling Papa Rudolph in the immensly popular "Sune" series. To put it in terms for a more international audience, it's basically like seeing Fred Rogers playing a Complete Monster serial killer. And doing it superbly!
Real-Life Relative: In the Swedish film of Dragon Tattoo, Lisbeth's mother Agneta is played by Noomi Rapace's mother, Nina Norén.
The use of Enya's "Orinoco Flow" in the American film was suggested by Daniel Craig. It was initially met with disbelief and doubts with regards to whether or not it would work. The rest, as they say, is history.
Rooney Mara told David Fincher that Lisbeth Salander was a natural redhead who dyed her hair black. Thus they created a red merkin for her to wear.
Development Hell: Sequels for the film got stuck in development hell for so long that Sony decided to ReBoot the next film with an entirely new cast. It is scheduled to be released in 2018.
According to director David Fincher, Rooney Mara dove headfirst into her portrayal of Lisbeth, chopping off her long brown hair to a dyed-black pixie cut, bleaching her eyebrows, getting both ears pierced (all together four times), and getting her eyebrow and one nipple pierced as well (nose and lip piercings were fake). Say what you will about her, she doesn't do things in half measures.
Daniel Craig also gained weight to make himself look more normal and less like James Bond.
Enforced Method Acting: The physicality of shooting the rape scene was so severe that the bruises on Rooney Mara's body in the shower scene were real. Her make-up artist was even fooled into thinking her make-up had already been applied.
Fake Nationality: All over the American film version. The only actual Swede in the main cast is Stellan Skarsgård. The rest are British, American, or Canadian, except for one Dutch man, and one Croatian.
Mean Character, Nice Actor: Yorick van Wageningen was so disturbed after shooting the rape scene with Rooney Mara that he spent a whole day locked in his hotel room, crying.
Playing Gertrude: Per Myrberg is only seventeen years older than his on-screen daughter Geraldine James.
Remake Cameo: David Dencik - who plays the young Morell - had played Janne in the original Swedish film.
Mikael catching the water bottle was an ad-lib by Daniel Craig.
While filming a diner scene, the crew discovered Michael Nyqvist's daughter working there while attending school. So they wrote in a couple of lines for her. She plays the barista giving Mikael a sandwich at the beginning.
Wag the Director: Mikael having an English accent was because Daniel Craig thought the idea of American, Canadian and British actors using bad fake Swede accents was silly.
There was a campaign among fans to get Noomi Rapace to reprise her role in the American remake too. Roger Ebert even championed it. She declined, having had enough of playing Lisbeth for three years in a row.
Johnny Depp, Viggo Mortensen, Brad Pitt and George Clooney were considered for the role of Mikael Blomkvist. Daniel Craig also turned the role down initially because he wouldn't be able to commit with Skyfall. However when filming on that was pushed back, he became available and took the role.
Max von Sydow was the original choice to play Henrik, but he had to drop out and was replaced by Christopher Plummer.