Spoiler alert! The following story details a major plot point in the new X-Men movie "Dark Phoenix," so beware if you haven't seen it yet.

“Dark Phoenix” was pretty much non-stop roiling emotions for Sophie Turner as she filmed the epic breakdown of X-Men heroine Jean Grey. So much so that a running joke arose between Turner and director Simon Kinberg.

“He would be like, ‘All right, another intense climactic scene for the movie!’ And it would be every single day,” says Turner, 23. “I was constantly draining myself emotionally, but it was so exciting for me because it's one of the first roles I've had where I didn’t just stand in the background, able to hang out and make jokes and (mess) around on set. It really put me to the test and I loved it.”

But one scene almost broke Turner.

Seriously. This is your last chance to bail out, because things are about to get super-spoilery.

“Dark Phoenix” finds Jean being blown up during a space mission, but instead of dying, she's possessed by a cosmic force that gives her god-like powers. Unfortunately, as addictive as these abilities are, this new entity inside her tears her apart, inside and out.

Jean was orphaned at a young age and taken in by Charles Xavier (James McAvoy) after she survived a car accident that killed her parents. But she finds out that Xavier’s been lying and her dad is still alive, not wanting to have anything to do with her after blaming Jean for his wife’s death.

The X-woman goes to her old home to confront him, and she runs into Mystique (Jennifer Lawrence) and the X-Men who’ve traveled to find her before she does something rash. Mystique tries to calm her down, but feelings get out of hand and Jean unleashes a telekinetic blast that forcefully impales Mystique onto the business end of a bulldozer, killing her.

“It's the catalyst for everything that happens in the movie," says Turner, who was “hyperaware” of how important Lawrence’s fatal finale was. “There was a lot of buzz around the scene, and I didn't take it lightly.”

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The problem was, Turner didn’t sleep at all the night before because she was so worried about the scene, and the actress had trouble mentally getting into it during filming.

“It's such a kind of emotionally hyped-up scene,” Turner says. “I was crying because I couldn't get it, and then the minute I started to feel that kind of frustration and that anger, I realized, ‘This is what Jean is supposed to be feeling right now.’

“The fact that I couldn't get the scene right made me get the scene right.”

Co-star Michael Fassbender was “really blown away” by Turner’s maturity and how she handled each shooting day, he says. “She really has to go to some pretty extreme places emotionally.” And Kinberg adds that actors would tell him how "incredible" she was: "Everybody knows she's a great actress from ‘Game of Thrones,’ but she definitely surprised all of us."

Headlining “Dark Phoenix” was “a daunting task,” Turner says, but she adored the support from Lawrence and others.

‘I felt like, ‘Well, I'm really going to (mess) this up because I have (freaking) Oscar winners and Oscar nominees all around me and they choose me to lead it? That girl from that TV show?’ ” Turner recalls. “To have them root for me was just the most special thing.