Warning: This article contains spoilers from Stranger Things Season 4 Volume 1.

When Matthew Modine first met Millie Bobby Brown, she was 11 years old — and in a cell. 

That’s not as strange as it sounds. Brown was on set for Season 1 of Stranger Things, where she portrays Eleven, a telekinetic young girl raised in the prison-like Hawkins Laboratory by a paternal yet manipulative scientist, Dr. Brenner (Modine), whom she calls Papa. Brown was already in character — dressed in a hospital gown with her hair shorn — when show creators Matt and Ross Duffer brought Modine over to say hi.

“The Duffer brothers were so excited for me to meet this young, new talent — they were giddy,” 63-year-old Modine tells Tudum. “They were so happy with the work that she was doing... ” He pauses to cast a playful look at Brown. “And I’ve worked with a lot with animals, so I knew how to work with young children—”

“You know that’s what they say about the older generation, too,” Brown, now 18, pipes up teasingly. “You’ve got to really take care of them—”

“Bring them a biscuit,” adds Modine.

“Give him some milk,” continues Brown. “Let them talk about back in the day.”

The co-stars dissolve into laughter. Finishing each other’s sentences isn’t unusual for Brown and Modine, and they continue to do so, sitting side by side, throughout Tudum’s cover story interview. The duo have maintained a close friendship since that fateful cell-set encounter in 2015, partly because their roles depend on it. 

See more from the Stranger Things Tudum cover shoot here

Considering the, at times, disconcerting nature of the characters’ relationship — Brenner is, after all, trying to exploit and weaponize Eleven’s powers — the actors quickly realized they needed to establish a strong connection. Not only did this mutual respect ensure that their on-screen dynamic was authentic, it also kept Brown and Modine’s emotional well-being in check. “Papa and Eleven have such an intimate relationship, so we needed to be able to bond to portray that,” says Brown. “We meshed, and it was just a true, real friendship. Nothing was ever forced. It just worked.” A lab experiment gone right, if you will.

“Matthew and Millie are so close outside of the show,” says co-showrunner Ross Duffer. “[In] Season 1, we’re hinting at Brenner, but we really get to know him a lot better this season. They had so much fun working on scenes together. It really was a blast.”

During that first season of filming, Modine grew so close to the Brown family that he kind of moved in with them. What was supposed to be a three-day stay in the family’s guest house turned into a three-month stint. “We renamed the guest house Modine House,” says Brown, whose mom would make dinner for the two of them after a long day on set. “If Modine was tired, we would just take the golf cart down to the house, give him his food and leave a note that we loved him.” 

Eleven and Papa throughout the seasons

Soon after, the bona fide buddies were heading out on family road trips, driving from Georgia to Florida. Along the way, they stopped off at a grocery store for treats where fans ogled the pair, surprised to see the two actors hanging out. “I think the whole supermarket was like, ‘What are they doing together?’” says Brown. “We were taking selfies in the cereal aisle — I still have those.”

Despite the on-screen tension between Eleven and Brenner, Modine has found that fans aren’t actually against the father-daughter dynamic. “They love the relationship between Papa and Eleven,” says the actor, whose son and daughter both work in Hollywood. “Obviously people are protective of Eleven, and they’re like, ‘You better not hurt her in Season 4,’ but if we [just] had an antagonistic relationship, I don’t think it would have the dynamic that it does. These two people are in a very complicated relationship of love. There’s a paternal instinct to help and protect her.”

We first glimpse that complex dynamic back in Season 1, but truly exploring the history between Papa and Eleven means returning to lab life in Volume 1 of Season 4. Other than a brief appearance in Season 2, Modine’s been mostly absent from the series since Brenner’s presumed death in the first season. When the Duffers asked him to reprise his role for the fourth season, however, he was in talks to do To Kill a Mockingbird on Broadway. After an incredulous reaction from Brown (“I was like, ‘Wait, what?!’”) and a sitdown lunch with executive producer Shawn Levy, Modine knew he had to return. As Brown puts it, “The slogan [of Season 4] is ‘The beginning of the end.’ We need to go and dig deep into Eleven’s past — and there’s no past without Papa.”

Or future, really. Having lost her powers after the Starcourt Mall showdown, Eleven’s only hope to regain her world-saving abilities is to trust Brenner. Once again, our young heroine finds herself underground at a top secret facility, undergoing sensory deprivation treatment in an effort to reboot her powers by reliving her trauma. Not exactly an easy feat when her memories include otherworldly-slash-deadly creatures, scenes of brutal murder and Brenner’s words, “What have you done?” echoing around her. 

“He’s having to force Eleven to go through her past and re-see things,” says Modine. “And unfortunately, along the way, there are some things that are not truthful, that are part of her memory and not actually part of her past.” As the Volume 1 finale reveals, another Hawkins Lab inmate — who’s evolved into Season 4’s big bad, Vecna, played by Jamie Campbell Bower — is actually responsible for the bloody flashbacks Eleven experiences, but Brenner never sets her straight. “That’s the heartbreaking thing,” says Modine. “That’s the cruel blow.”

As Eleven revisits her childhood with Brenner, the memories vary from instances of real affection to darker moments of true exploitation. To keep from being overwhelmed during those takes, Modine and Brown established a system where they would blink once at one another to check in and say “I love you” amid challenging scenes.

They also spent time establishing more of their characters’ backstory together that we don’t see on screen. “You have to build ‘memories’ for that kind of relationship,” explains Brown. “We collaborated because it needs to be believable. We built gestures, like Papa booping my nose and things like that.”

Beyond the endearing nose boops, Season 4 has been incredibly physical for Brown, who spends a decent amount of time being dragged across set on wires. In true protective-parent fashion, Modine advised Brown early on to always remember that her body doesn’t know when she’s acting. “I do a lot of physical and emotional stuff in the show, and when my heart rate goes up, my body thinks that it’s going through something,” she says. “So we have to really take care of our bodies and give them that healing that they need.”

Looking forward to Volume 2, both are tight-lipped, but Modine will share that an upcoming “emotional” scene between them was the only time he had ever seen the Duffers step in to give Brown “a really specific direction.” The Duffers add that they were “blown away” by the duo’s performance in Episodes 8 and 9. “They have some scenes together [in Volume 2] that are, I think, the best acting either of them have ever done,” says Matt Duffer.

With Season 5 confirmed as the last in the Stranger Things series, on-location quality time is limited for Modine and Brown. But regardless of where their character arcs take them, they plan to stay in each other’s lives. “I found that person to rely on and know he’s always got my back,” says Brown. So with all they’ve experienced together, what would their dream collaboration be? “Birdy 2,” jokes Brown, throwing it back to Modine’s 1984 classic. “Except we are birds. Or I’m going to be Scout in To Kill a Mockingbird on Broadway. I will audition for you.” As the saying goes, Stranger things have happened... 

Danny Kasirye

Watch Stranger Things Now on Netflix