>DARK PHOENIX was greenlit as a two-parter. The first movie would focus on Jean losing control of her powers, with the X-Men fighting to protect her and the Brotherhood of Mutants trying to kill her, and end with the battle in the streets of New York. The second part would focus on Jean’s transformation into the Phoenix and the arrival of the Shi’ar to kill her before she threatens the entire universe, and end with a massive battle between the X-Men and the Shi'ar armada in the United Nations.
>The two movies were to be filmed back-to-back, and the plan was to film the main action setpieces first. So the United Nations Battle set, the largest of any X-film, and the New York Street Battle set were all being built.
>Two months before principal photography, Fox and Disney began negotiating the merger. Fox slashed the project's budget and told Simon Kinberg and his team to scale it down to one movie. They had to rewrite the entire script, condensing pieces and ideas from the old drafts while cutting out characters, storylines and setpieces.
>Kinberg realized it would be a mistake to include the Shi'ar in the new script, as he wouldn't be able to explore their origin and they would steal the spotlight away from Jean, so they were changed into generic alien invaders, who were made into shapeshifters pretending to be human to save costs on CGI/prosthetics.
>Jessica Chastain signed to play Lilandra, the empress of the Shi'ar. She was an antagonist, but not a villain, and wanted to destroy the Phoenix to protect the cosmos. Her relationship with Xavier would be explored, and she would part on good terms with the X-Men. When the scripted changed, they had to delay her from shooting while they figured out who she would play and how she would be involved. Chastain started two and a half months after all the other actors had started.