Jessica Chastain's role in X-Men: Dark Phoenix has been a mystery since she was first seen in the film's trailer. The only information fans were given was that she was playing someone named Smith who is an "otherworldly shapeshifter." The description is pretty accurate, but there's definitely more to the story! The truth is that Chastain actually portrays two different characters in the final installment of the Fox franchise, and one of them is pretty well-known to X-Men comic book readers.
Warning, spoilers for X-Men: Dark Phoenix are ahead!
When she first appears on screen, Chastain plays Margaret, a sweet woman hosting a dinner party who ventures into her backyard to investigate what's bothering her agitated dog, Luna. When she gets out there, she's attacked and (presumably) killed by the aliens who were hiding among the trees. As we learn later in the film, the aliens are survivors of the D'Bari Empire who are driven from their peaceful existence when the Phoenix force consumes their sun and destroys their solar system, bringing their race to the brink of extinction. The alien who takes Margaret's form is known as Vuk, the leader of the refugees, and she is not interested in making friends with anyone on Earth. (Vuk is depicted as male in the comics, but since the alien is wearing Jessica's skin, we'll refer to them as "she.")
When Vuk encounters Jean Grey (Sophie Turner) post-Phoenix connection, she encourages the young mutant to focus on controlling her new power, telling Jean that she can use the power to rebuild worlds. In reality, Vuk is planning to use the power to resurrect the D'Bari race and begin again, killing Jean and all of humanity in the process. Jean is able to overpower her and dispatches the remaining D'Bari, but it's at the cost of her own life.
In the comics, the D'Bari aren't shapeshifters like they are in the film and Vuk isn't even on D'Bari IV when Phoenix destroys it. He does attack her to avenge his decimated race, but it's much later after the destruction of his home planet. Jean manages to telepathically convince Vuk that he's killed her, which leads him to leave Earth in victory. It's a much more peaceful ending for the alien than their ending in Dark Phoenix, that's for sure.
James McAvoy, who plays a more cynical version of Charles Xavier this time around, revealed to YahooUK that the film underwent drastic changes during reshoots and Chastain's character took the brunt of the changes. "The end changed a hell of a lot," McAvoy said. "The finale had to change. There was a lot of overlap and parallels with another superhero movie that came out . . . a while ago."
"My character changed a lot, which is an interesting thing because I'm not playing someone from the comics," Chastain added. "So it was always every day trying to figure out, 'Who am I? Who is the mystery that is this character?' And then understanding with the reshoots, 'Oh, it's changing again.' It was a constant evolution . . . So yeah, my character changed."
Considering that the original iteration of the D'Baris story (seemingly villainous aliens who are actually refugees seeking a new home after theirs is torn apart) is pretty similar to the Skrulls' story in Captain Marvel, we're going to assume the Marvel film moved the Dark Phoenix crew to change things up. It does make you wonder what kind of character Chastain was meant to portray before and how that changed the final act of the film as a whole. We'll probably never know, but it's definitely food for thought!