In an unspecified draft by David Hayter, Dan and Laurie still spend the night at Karnak after Veidt assassinates the leaders of almost every country. However, Laurie wakes up later and then gets seriously drunk on champagne, possibly because of the loss of Jon and Rorschach. She then discovers that Dan is fighting with Veidt which ends with Dan killing Adrian Veidt with a boomerang.
Hugh Jackman took ice cold showers every morning of filming in order to help get into character. This tradition started when jumping into the shower at 5 AM before realizing there was no hot water. Shocked awake but not wanting to wake his sleeping wife, he grit teeth and bore it, before realizing that this mindset - wanting to scream and lash out at something but having to hold it in - was the mentality that Wolverine is in constantly. He's taken cold showers for every Wolverine film ever since.
The scene in the train station where a young boy smiles at Cyclops and he smiles back was unplanned. The boy was a huge X-Men fan, and Cyclops was his favorite. The scene originally called for Cyclops to look at the train schedule, but according to Bryan Singer the boy could not stop smiling at James Marsden. Finally, during one shot, Marsden just looked back at him and smiled, much to the boy's delight. Bryan Singer liked the idea so much, he kept it in the film, and told the actress playing the boy's mother to react the way she did.
The Mansion used as the Xavier school, is the same one Billy and his family lived in in Billy Madison (1995) and as the Luthor Mansion in Smallville (2001)
In 1994, screenwriter Andrew Kevin Walker wrote a draft of the script: the X-Men (Professor X, Cyclops, Jean Gray, Beast, Iceman, Angel and new member Wolverine) must stop the Brotherhood of Mutants (Magneto, Sabretooth, Toad and new member the Blob) from conquering New York City, while at the same time are set upon by a triplet of Sentinels, robots created by anti-mutant government officials Peter Henry Gyrich and Boliver Trask. The script focused on the rivalry between Cyclops and Wolverine, and had Magneto the cause of the Chernobyl disaster; also included was the X-Copter and the Danger Room. The script was never used, but dialogue/plot elements were used in the film's official novel adaptation.