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 admitted that she's very unlikely to ever reprise her role as Jane.
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."
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.
Genius Bonus: 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.
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/Shrug 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.