'You' 's Tati Gabrielle Says Penn Badgley Morphed into 'a Whole New Person' on Set During Tense Marienne Scenes

"As Tati, I couldn't feel Penn anymore," the actress tells PEOPLE

Photo: Cindy Ord/Getty, Ilya S. Savenok/Getty

Warning: This post contains spoilers from season 4 of You.

Tati Gabrielle got to see a whole need side to costar Penn Badgley while filming You.

Viewers last saw Gabrielle's character Marienne Bellamy early on in season 4 escaping former beau Joe Goldberg (Badgley) after he stalked her, tracking down her in Paris. And while fans initially believed he left her be thereafter, Joe actually kidnapped Marienne and trapped her in a glass cage near his new London residence. What's worse, Joe had no idea what he had done until it slowly began coming back to him — he completely blacked out during his interactions with Marienne after Paris.

Marienne had been stuck in the cage for several weeks by the time fans see her again, but a skeptical student Nadia Farran (Amy-Leigh Hickman) of Joe — a.k.a. Professor Jonathan Moore — discovered her there. (Nadia had been on to Joe as viewers soon discover he, in fact, was the "Eat the Rich Killer," not Ed Speleers's character Rhys Montrose)

And as intense as those scenes of Marienne trapped in solitude were to watch, Gabrielle exclusively tells PEOPLE they were just as hard to film across six- to 10-hour shoot days occurring between two to three weeks.

"I got chills immediately within the first few seconds of me being in there and then this crazy feeling of anxiety just started rushing through my body. I feel like it was almost like a trauma response," says Gabrielle, 27. "I personally have never been in the cage. I had already started doing work with Marienne in my head. And then, I think just my brain as Tati, knowing what the cage is and what happened in there, I couldn't stay in the box for more than 30 seconds the first time I stepped into it. I was like, all right, I don't want to be in here until I've got to be in here."

Courtesy of Netflix

The Chilling Adventures of Sabrina alum continues, "Once we actually got into the work, it was definitely a challenge. I mean, just one doing, especially that first episode and episode eight, being by myself, it was the first time, as an actor, I had to sort of carry some 20 minutes, basically, worth of work by myself. Nobody to bounce off of. I was like, OK. But it was a really fun challenge. I feel like I really got to grow as an actor in that way."

The actress eventually got used to filming in the cage, even feeling as if the place was her "own personal studio apartment." However, she admits something else that's taken her by surprise while filming her entire stretch on this series is how "wild" it is to see Badgley, 36, essentially morph into Joe's dissociative state.

"So in season 3, I will never forget the first time me and Penn did our first scene. And we were talking Penn and Tati, and then they got ready to call action, and Penn's eyes changed. Just a whole new person stepped into his body. And I was like ... Oh my God. He's so good," she recalls.

"He started doing these scenes, and Joe, in this dissociative state, it was sort of doubling anxiety-inducing because, on one hand, Marienne is desperate to get through to Joe," she further explains. "He knows that something's happening mentally speaking, and she's trying to pull him back out. And then on top of that, as Tati, I couldn't feel Penn anymore."

JOHN P. FLEENOR/NETFLIX

But Gabrielle says witnessing Badgley in action this way was "such a testament to his freaking talent."

"Both Penn and Joe were dissociating. And I was like, Oh no, where'd you go? It really made [for] just very authentic and very genuine moments," she continues. "And that's why I love working with Penn throughout this process is because he's such a present scene partner for whatever the case is, and as well as even off set, always willing to talk about something, work through something. And he was a big supporter for me through my time in the cage."

Marienne eventually escaped the cage by pretending to overdose on pills. As Joe left her body on a park bench, Nadia came to the rescue and helped Marienne quietly return to her child in Paris.

JOHN P. FLEENOR/NETFLIX

As for what the future holds for Marienne, Gabrielle admits: "I don't know. That's definitely a question for Netflix, and [You co-creator] Sera Gamble."

"But I would love to see Marienne come back because I feel though she was able to get out alive and be with her daughter, and she is able to live a peaceful life now, Joe thinks she's dead," she explains. "But I feel like something just still doesn't sit right with her in her moral compass, and the fact that he is still out there and is not facing any consequences for the things that he's done."

Adds Gabrielle, "I would be interested to see Marienne come back and break Nadia out and go and do the thing. Yeah. But we'll see."

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All four seasons of You are streaming in full on Netflix.