Chris Hemsworth grew out his hair for over a year to have more authentic long hair, rather than use a wig, like he had to do in Thor (2011).
(at around 1h 23 mins) Chris Hemsworth improvised hanging Mjölnir (his Hammer) on a coat hook in a polite manner, after playing with it between takes.
(at around 1h 5 mins) Thor accidentally destroys a statue of his grandfather Bor, and Loki wisecracks that he killed him. In the Marvel comics, Thor ended up killing his grandfather as part of a deception by Loki.
Final film of screenwriter Don Payne, who died eight months before its release. This was his third Marvel film, after Fantastic Four: Rise of the Silver Surfer (2007) and Thor (2011).
The prologue was filmed in a blend of live-action and CGI, as the Asgardian/Dark Elf costumes were too constrictive in which to fight effectively and convincingly. There are only three characters played by human actors in the entire scene, Malekith, Kurse, and Bor. All other characters are CGI.
Stan Lee: (at around 52 mins) As the man who asks Dr. Erik Selvig (Stellan Skarsgård) for his shoe back, when Dr. Selvig is in the mental ward explaining the Convergence theory.