MOVIE URBAN LEGEND: X-Men: Dark Phoenix had its original ending changed because it was too similar to Captain Marvel's ending.
The recent release of Dark Phoenix came with it stories about how the final third of the film was almost completely reshot. Writer/director Simon Kinberg had been speaking about how the change to the ending was to make it more of a grounded ending (as the original ending took place in outer space). However, James McAvoy then took things in a different direction when he noted in an interview with Yahoo Movies UK (alongside Michael Fassbender) that
“The end [of Dark Phoenix] changed a hell of a lot.”
“The finale HAD to change. There was a lot of overlap and parallels with another superhero movie that came out... a while ago. And we had no idea that we were...”
“They had spies on set,” interjects co-star Michael Fassbender, returning for a fourth time as Erik “Magneto” Lensherr, “and [they] basically stole our ideas.”
“We were basically trawling through the source material it seems,” finishes McAvoy.
Since Captain Marvel's ending was in outer space (and it also dealt with shape-shifting villains just like X-Men: Dark Phoenix), many fans assumed that Captain Marvel was the film in question.
The timing, though, never quite matched, since the Dark Phoenix re-shoots occurred in the end of 2018, when the only information anyone had about Captain Marvel was the trailer that was released in September 2018. Now, the trailer itself showed enough that you could believe that Kinberg might have wanted to change the look of Dark Phoenix to make her look different from the effects used in Captain Marvel, but otherwise, it seems unlikely that they would have been aware of the specifics of the ending at the time that Dark Phoenix was doing reshoots (which was before Fox was purchased by Disney, by the way).
Kinberg, though, elaborated that it was Captain America: Civil War that was the film that they felt was too similar. He explained to io9, "My original ending didn’t have the entire X-Family together the way they are in the film now. More than Captain Marvel, you could see a lot of Civil War in that ending. Usually, these big, huge action movies have the climactic moment in the third act. I loved the way that Civil War had its big action action set piece where everyone’s facing off more towards the end of the second act rather than in the third, so that after that huge battle, you’re left with Winter Soldier, Captain America, and Iron Man."
Again, though, Kinberg noted the visual similarities between Dark Phoenix and Captain Marvel, "It’s this telescoped down view of their raw emotions, and I loved how intimate that was. That’s what I was going for with Dark Phoenix’s ending even though it then might have looked like Captain Marvel for about two minutes."
We heard that in one cut, Jean Grey dies, which wasn’t received well. But overall, the major ending change-up, executed in reshoots, entailed going from an intimate ending with Jean Grey (Sophie Turner), Tye Sheridan (Cyclops), and Charles Xavier (James McAvoy) against Jessica Chastain’s Vuk. The feeling from the audience in testing was that they wanted to see all the X-Men heroes fighting in the end. There have been reports out there that the ending was changed-up because it was too similar to Captain Marvel. This isn’t true: No one on the Fox/Dark Phoenix production side had any intel of what Captain Marvel would be like before it was released.
I think that there is enough there to dismiss the idea that the ending was changed because of Captain Marvel.
Thus, I'm going with the legend as...
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