This article contains spoilers for the current season of Stranger Things.

During an interview with Variety, Millie Bobby Brown sat down to discuss Eleven’s journey in Stranger Things 4, such as what it was like filming Eleven reliving her early years with the Nina Project. Most of the scenes utilize footage of Brown, but some shots involved a younger stand-in, Martie Blair from The Young and the Restless. Brown worked extensively with Blair to capture how the young Eleven would act.

“It was really important for me to help her through that because I didn’t really have anyone helping me through who Eleven was going to be. I would come to set before my scenes and direct her through everything. I would hide behind the wall and scream with her when she got anxious about screaming to do my powers. I would help her with some of her facial twitches and things like that — very specific things that no one probably would even tell, but me watching the show, I wouldn’t be able to live with myself if I didn’t help her through it."

After they finished shooting, they inserted Brown’s face over Blair’s during post-production. In order to sync everything, Brown had to do this process called the Lola machine. For three days she worked in this room and did all of Blair’s scenes, except with just her face. According to Brown, “it was a really interesting process — definitely once in a lifetime.”

“I mean, it always shocks me what we can do in our show. I was so pleased with the outcome. And, you know, Martie’s performance was amazing. They just had to add my face to it. That mixture of all of us working, collaborating together — it was so weird, so different.”

A challenging part about filming the flashback sequences was having to be inside the Nina tank. Brown had to lie in a saltwater tank for 10 to 12 hours a day for a few days. Everyone was moving around her, she couldn’t get out, and she couldn’t hear anyone. So when Brown would get notes from a microphone, she would just hope she was doing what maybe they were saying.

“Also, I suffer from claustrophobia, so when we closed the tank, I definitely felt like, ‘Oh my gosh, I am Eleven, I am really going through this.’ Obviously, what she has to go through, reliving those memories, was probably harder, though, than a tank.”

Related: Stranger Things Season 4: Where Is Kali and Will Eleven’s Sister Return?

Millie Bobby Brown on Being Young Again

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Brown was also asked about what it was like putting herself in the mindset of Eleven at that age. As Brown explains, Eleven was raised in an asylum with a man that she isn’t biologically related to but emotionally and psychologically attached and connected to, so she suffers from severe Stockholm Syndrome. Something that Brown wouldn’t have known at ten years old.

“But now being 17, going back to that 10-year-old self, I’ve learned all of this new information about Eleven. So I got to bring a whole new dynamic and perception to Eleven that I had never been able to explore because of how young I was. This person who hurts her, Papa, he also loves her the most. And facing him again, in this relationship that is so destructive and so painful, was one of the hardest things I’ve had to do within the show.”

The hardest thing Brown has had to do as a young actor, not just for the show, but her career, was to portray Eleven recovering from dissociative amnesia. A scene that coincides with Jamie Campbell Bower’s orderly character, newly revealed to be villain Vecna, giving this monologue explaining his character’s motive and asking Eleven to join him.

“She really has to remember something her brain has protected her from. So this conversation, she actually doesn’t remember because our brain protects us from it. I have to almost pretend I’ve already gone through it, which was also very hard, and then relive it again. It’s so emotional, and so confusing, because this tiny person is being told all of this information and almost being asked to join him on this evil and demonic journey that he wants to go on.”

Vol. 2 of Stranger Things 4 will arrive on July 1st. It will be followed by a fifth season, which will also be the show’s last. When asked about what she would like to happen during the final season, Brown responded that she’s going to leave that up to the Duffer Brothers because they’ve always done everything she wanted for Eleven.

“I know it’s a really boring answer, but I don’t think about it. Soon as I get on set, they tell me what I’m doing, and that’s it. Let the geniuses be geniuses and let the actors act.”