Rebecca Romijn's (Mystique's) and James Marsden's (Cyclops') roles were reduced substantially when this movie was rushed into production, and the two cast members had prior scheduling conflicts.
Budgeted at $210 million, this was, at the time, the most expensive movie ever made, surpassing King Kong (2005), which cost $207 million.
Kelsey Grammer was so keen to play Beast that he agreed to do an audition. This was the first time Grammer had auditioned in over twenty years.
When Bryan Singer was going to direct, he and his writers Dan Harris and Michael Dougherty wrote a treatment solely based on the X-Men storyline "The Dark Phoenix Saga". The deceased Jean Grey returns, with a new, more destructive personality called The Phoenix. She would be manipulated into joining the Hellfire Club, by their telepathic leader Emma Frost (Sigourney Weaver was intended for the role). A three-way battle occurs between the Club, who want to take over the world, the X-Men, who want to save their comrade, and Magneto's Brotherhood, who want the Phoenix for their own plans. At the end, to save everyone, Jean kills herself, but her spirit lives on and transcends into a divine being, which Dougherty compared to the star child in 2001: L'odyssée de l'espace (1968).
For the opening de-aging effect, Sir Patrick Stewart and Sir Ian McKellen gave the visual effects experts pictures of themselves as younger men, so that the computer program "Lola" could make them look younger. This was the first film to ever use de-aging.
Stan Lee, Chris Claremont: (at around 2 mins) The X-Men creator and longtime X-Men writer appear in the opening scene, as neighbors of Jean Grey. They are credited as "Waterhose Man" and "Lawnmower Man", respectively. (Claremont created the "Dark Phoenix Saga" in the X-Men comics, which was the basis of the Jean Grey and Phoenix storyline in this movie.)