Most of the extras playing frozen people were actually mimes, who are used to not moving.
During filming, Hugh Jackman's sister visited the set. As a joke, she was made-up in Jackman's full Wolverine costume and make-up, and walked around the set. Apparently, she was very convincing as Jackman. Bryan Singer was not aware that it was not Jackman in the Wolverine costume, and commented of her work during shooting that "Hugh is acting very strangely today."
Professor Xavier's wheelchair from X-Men (2000) was bought by a lawyer who also worked for the same law firm as Sir Patrick Stewart's attorney. When production began, the studio realized they had no chair anymore, so the lawyer rented it back to the studio, as Stewart said in an interview, "for a significant sum."
The set for Stryker's underground lair was built in an old Sears warehouse, and was the largest set in North America. It involved over sixty miles of cable, and was so large that cast and crew members used bicycles to get them to and from the bathroom as quickly as possible. Even then, the production only used about half the space in the warehouse.
Unlike the first movie, where Rebecca Romijn wore contact lenses, this time, Mystique's eyes are rendered digitally.
Bryan Singer: (at around 31 mins) A security guard, in a scene where Professor X and Cyclops go to see Magneto.