If there’s one person you could trust with your deepest, darkest secrets it would be Sadie Sink.

The 24-year-old actor learned how to keep details under wraps during her years starring on Netflix’s hit series Stranger Things. And she’s become a real expert after keeping her lips sealed about her role in Spider-Man: Brand New Day for two entire years.

[Warning: Spoilers from Spider-Man: Brand New Day ahead.]

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Now that Spider-Man has swung into theaters (earning more than $1.18 billion worldwide in the process, and officially becoming the highest-grossing movie of the year), Sink can finally discuss her role as Jean Grey, a telepathic mutant and core member of the X-Men. She’s one of the most important and powerful X-Men, and was played by Famke Janssen and Sophie Turner in previous films. “I’m stopping myself from saying stuff and then I realize, Oh wait, no, I can talk about it now, which is awesome,” she tells Vanity Fair.

In the film, Sink’s character is introduced as a mysterious entity who can somehow teleport her consciousness into different bodies, controlling them as she attempts to break into a government building to find her missing sister. She’s an adversary to Spider-Man (Tom Holland)—but as anyone familiar with the character knows, she’ll be a leader of the X-Men in future films. Marvel is currently assembling a new cast for its upcoming X-Men movie, which is planned for 2028 and will be directed by Jake Schreier. Sink and the other X-Men will likely appear in other Avengers movies as well, though the details (you guessed it) remain secret.

In one of her first interviews about the role, Sink tells Vanity Fair about how she learned she was up for the job from online rumors, if she ever got close to slipping up about her role, and why it was a “no-brainer” to say yes to the part.

Vanity Fair: How did this role first come to you? Was there an audition process?

Sadie Sink: No. It kind of started with these online rumors I was seeing. Usually there’s not much truth to those, but in this case there was. My team heard from Marvel and Sony for Spider-Man. And then I met with Kevin [Feige] and Louis [D’Esposito] from Marvel, and they told me about the character and about what my involvement would look like. I was unfamiliar with X-Men and Jean Grey at that point, but they kind of broke it down for me. And then it just kind of happened.

I didn’t have a script or anything. I don’t think I got a script until I arrived in London to shoot it. But the more that I learned about the character, the more I just got really excited about it and excited about this version of her too, which just feels really complex and different from what we’ve seen.

What was exciting to you about her?

All the women in X-Men are so strong and interesting—their relationship with their powers, how they’re capable of doing incredible things. It gives them so much strength, but then it also affects their lives in negative ways. It was kind of nice to start out with the negative side of it with Jean, how she’s just learning what she’s capable of. And before she can even develop her relationship to her powers and what they mean to her, they are used by people who want to do harm with what she’s capable of, what her sister’s capable of. I was excited to not play a villain, but just play someone who’s only been hurt by having these abilities, and so she only knows how to use them as a threat or to defend herself.

Did you end up going back to watch the other movies that have this character, whether that was Famke Janssen or Sophie Turner?

Yes, I did an X-Men deep dive and I watched all of it. I was also really interested in, what do her powers look like? I’ve spent 10 years on Stranger Things, so I’ve been around that a lot. But that was maybe the thing I was most nervous about: How do you play that? I looked at what the other actors were doing physically, and of course visual effects play a big role in that. But I think it was most important to me that, as with anything, when you’re acting, you’re bringing a huge part of yourself into the role. So it’s going to be inherently different from what we’ve seen. It feels like a very different version than what we’ve seen before.

Performing telepathy seems very hard, because it’s all internal.

I was like, Whatever I decide now, I have to carry that with me a little bit. And so it was more important for it to feel almost invisible, because her whole thing is that she uses her powers to be invisible. You don’t really notice it—it’s just more in the eyes.

Joining Marvel can be a yearslong commitment. Did you have to pause and think about if you wanted to be committed for that long?

Of course, with anything you do have to stop and consider. For me, it felt like a character that would have a lot to offer over however many times we see her. And also the idea of returning to something or having a consistent character that you’re playing is something I’m also familiar with. I like that structure. So it felt like a no-brainer to me. It’s a great character to spend some time with.

You’ve been on a show that had a lot of passionate fans with strong opinions. Does this feel different?

With Stranger Things, Max didn’t exist before me. So what fans are seeing of her, it’s all fresh and you don’t know what’s going to happen to her next. But with a character like Jean Grey, who’s been around for much, much longer, there’s so many stories to pull from, so many iterations of her. She means a lot to a lot of people. There is an extra element. People are protective over her.

Was there ever a moment when you came close to slipping up, or someone in your cast came close to slipping up and revealing the secret?

Not publicly, no. There were times where maybe I was like, Oh my God, have I told too many people? Did I tell the wrong person, or did someone hear me when I whispered it at that restaurant? There was definitely the paranoia of that. It’s nice for that to be gone.

Did you get any texts from friends after they saw the movie?

There were people from London, the play [Romeo & Juliet] I did there, and they were like, “I knew it!” And also friends of mine that I didn’t even know would be familiar with this character, but they are, and how excited they are and how knowledgeable they are about her.

Kit Connor was just reported as playing Scott Summers, a.k.a. Cyclops.

Did he? There’s always the rumors, and I just never know what’s official.

Are you testing with the other people they’re casting, or is it all happening sort of separate from you?

It’s a little bit separate. So I’m finding out through [director] Jake [Schreier]. There’s also so many online rumors, but it’s coming together. I think the timing of it is so exciting, and the way they planned it out is really clever just with introducing one of the mutants, a very important mutant through Spider-Man. I think a great cast is coming together, and I think people are going to be really excited about it when it’s all announced.

Scott Summers and Jean Grey have such an important connection in these movies. So I assume that’s a big one for you.

Yeah.

You’re really good at this. What else is coming up for you in the near future? The adaptation of John Proctor Is the Villain?

It’s still in development. We don’t really have dates for that yet. I’m excited about that, but I won’t be in it. I’m producing it. We’ll shoot X-Men, and then I’m going to do a series with FX and A24 called The Marriage Plot, so that’ll be next year. With Stranger Things ending, it’ll feel weird next year not being like, Oh wait, we got to shoot another season. We have to get ready to go back to Atlanta to shoot. It’s exciting to be able to have more room in the schedule.