Although not in the movie, Sir Patrick Stewart jokingly informed Bryan Singer that he would be willing to play Mystique, should Jennifer Lawrence "start playing up." He said: "I am so ready to be naked, painted blue, and the world is ready for it too."
Particularly with Psylocke, Nightcrawler, and Cyclops, the outfits and characters' looks were designed to pay homage to their comic book counterparts, differently from past films when the X-Men used standard black leather uniforms. Cyclops' costume is taken from Jim Lee's blue and yellow design in the 90s.
For the role of Charles Xavier, James McAvoy shaved his head. It was something he had actually done before starting work on X-Men: Primera generación (2011), before he learned that the producers wanted Xavier to have a full head of hair, forcing him to wear hair extensions.
This is the first time that the Summers brothers, Scott Summers (Cyclops) and Alex Summers (Havoc), appear together in a live-action movie. In the comics, there was another brother, Gabriel Summers (Vulcan), but he has not appeared in any shows or movies, and the filmmakers decided two brothers in a film was enough.
The film was to be subtitled "Age of Apocalypse" after the comic of the same name, but this was changed to "Apocalypse" for a more ominous title, as well as to avoid comparison with Avengers: Era de Ultrón (2015), Transformers: La era de la extinción (2014), and El secreto de Adaline (2015).