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."
Unlike the first movie, where Rebecca Romijn wore contact lenses, this time, Mystique's eyes are rendered digitally.
The replica of the President's desk in the Oval Office was so detailed that it took two months to build.
Bryan Singer: (at around 31 mins) A security guard, in a scene where Professor X and Cyclops go to see Magneto.