Despite being the main villain (and even being on the poster), Christopher Eccleston's name doesn't appear in most of the promotional material, including said poster.
The end credits sequence includes individual credits with headshots of most of the cast, including Alice Krige (who only shows up in one scene) and Chris O'Dowd (who appears in two) — but not Jonathan Howard, who played the much bigger and more plot-significant role of Ian.
Alan Taylor is not particularly fond of his experience on making the movie. He claimed that he was given a lot of freedom when he was filming only for Executive Meddling to change everything in post-production.
Idris Elba apparently didn't have a good time making the movie, and claimed the shoot was tedious and exhausting.
Natalie Portman wasn't too happy with this project from the start, to the point where she reportedly attempted to quit in protest of the departure of initial director Patty Jenkins, and only stayed on because it was less trouble than potentially getting sued for breach of contract. In mid-2016, she said that she was very unlikely to ever reprise her role as Jane, though she later mended fences with Marvel Studios, resulting in her eventually returning for Thor: Love and Thunder.
Christopher Eccleston has said Marvel misled him by neglecting to mention that he'd be required to sit in a makeup chair for 6-to-8 hours a day to have his makeup and facial prosthetics applied. He also admitted he only took the role in the first place for the money. He later described making the film as "Just a gun in your mouth."
Fandral and Hogun fight in the battle at Vanaheim.
Extended dialogue during Jane's date with Richard. She talks about her relationship with Thor and he talks about his break up with his cheating girlfriend.
Loki fantasizes of being crowned and ruling Asgard while holding Mjolnir, even casting an illusion of him doing just that, but his mother interrupts him telling him that he could not distinguish between reality and illusion.
Thor walks in on Frigga using her illusory skills to talk to Loki in prison. He discusses Loki with her.
Volstagg brags about his exploits during a dinner party.
Malekith learns that the Aether is been taken into Asgard. He informs Algrim to prepare for battle.
Foster wakes up in Asgard and talks with Thor. She sees children playing with a Magnetic-Propulsion Ball, and takes it, but ultimately returns it.
Foster learns about the Aether. She talks with Thor about his mother's death until they are interrupted by Tyr.
Odin shows Jane and Thor the book about the dark elves, including asgardians scientists working.
In an alternate version of Frigga's death scene, Odin allows Malekith to kill Frigga as he is unwilling to give up the Aether. Frigga begs Odin to kill Malekith and his army, knowing he has the power to do so but he chooses otherwise and she is killed.
Thor fights a group of Asgardian guards, but refuses to kill any of them.
A news presenter gives a report in the weird happenings in London, including flocks of birds going missing only to reappeared somewhere else.
Chris Hemsworth grew out his hair for over a year to have more authentic long hair rather than use a wig. (A particularly funny part of the Hilarious Outtakes is his hair entering Hemsworth and Natalie Portman's mouth during a kiss.)
Executive Meddling: Malekith had more of a backstory to tell but because of Tom Hiddleson's popularity, Loki received an expanded role in the plot, which resulted in this backstory being removed to save time.
Hemsworth, Alexander and Hopkins from the last movie.
Zachary Levi (Fandral) is American.
Focus Group Ending: This film was originally supposed to be Loki's last appearance, but the ending was changed due to negative reaction to his death at the test screenings. In the final version, Loki fakes his death and then casts a spell on Odin to take his place as the king.
Loki's Bond One-Liner to Kurse ("See you in Hel, monster."): it might not really be a reference to the more common Stock Phrase "see you in hell," but to the similar-sounding Hel (place for the dead in Norse mythology), ruled also by a person named Hel, who of all people is Loki's daughter in the original mythos.
Svartalfheim literally means "home of the black elves" in old Norse.
"Black Hole Grenades" for the Dark Elf bombs that open vortexes to suck people in.
"Friggason" for Loki because Frigga is the sole parent whom he still loves (and she's also the only parent who still loves him unconditionally). Moreoever, Loki is revealed to be his mother's son, as he had learned magic from her, they share a talent for deception, and their fighting styles are virtually identical.
Looping Lines: The scene where Thor and Loki are arguing on the skiff while flying through Svartalfheim had to be completely dubbed, due to the dialogue being inaudible because of the large fans on set.
Real-Life Relative: Chris Hemsworth's wife Elsa Pataky was Natalie Portman's stand-in and played Jane in the post credit scene.
Recursive Adaptation: Malekith was an obscure and forgotten villain from Thor comics. The film briefly raised the interest in him again, so he was used again as a regular villain. And, although the MCU Malekith turned out to be rejected by the audiences and never showed up again, the returned Malekith in comics proved to be quite a success.
Throw It In!: The gag of Thor hanging his hammer on a coat hook in Jane's apartment was an improvised bit by Chris Hemsworth who obviously thought that a warrior prince/god like Thor would be mindful of the proper etiquette of checking one's weapons when you enter another's home.
Alan Taylor wanted to have a Creator Cameo as the archer who lights Frigga's boat at the funeral, but he was turned down for insurance reasons.
An early ending of the final battle against Malekith had Thor concentrating lightning from all nine realms to destroy him and the Aether, since his method of using only one lightning like on Svartalfheim was ineffective. However, the filmmakers wanted all characters to contribute to Makekith's demise, so they added Jane, Selvig and Darcy using the gravity spikes to help Thor.
Word of God: Odin's fate had been given different answers by some of the crew. Kevin Feige didn't outright answer the question but said that there are plans for him, Alan Taylor thought he's dead but wanted to know how he would end up in the sequel, and Anthony Hopkins didn't have an idea and believed that he's dead. Thor: Ragnarok finally answered the question, Odin was not dead, Loki ensorcelled him and banished him to Earth. He does die during the events of Ragnarok though.
Thor: The Dark World is the only MCU movie not to have any scenes set in the United States.