[Spoiler alert: The following interview covers plot details from the entire eight-episode first season of Netflix’s “Stranger Things.” Do not read until you’ve finished the season.]

The world got very strange indeed over the first eight episodes of Netflix’s “Stranger Things.” The ’80s-set and inspired series from creators-writers-directors The Duffer Brothers (twins Matt and Ross) solved its central mystery of exactly what happened to Will Byers (Noah Schnapp) — who was reunited with his mother, Joyce (Winona Ryder), and brother, Jonathan (Charlie Heaton), after spending a week in a parallel dimension known as the “Upside Down” — but raised plenty of other questions.

What happened to telekinetic badass Eleven (scene stealer Millie Bobby Brown) after she destroyed the Upside Down monster? Does Chief Hopper (David Harbour) know where she is, or is he leaving those Eggos in the woods in an attempt to reach her? What sent Nancy (Natalia Dyer) back into the arms of her boyfriend, Steve (Joe Keery)? Can Mike (Finn Wolfhard), Dustin (Gaten Matarazzo), and Lucas (Caleb McLaughlin) possibly have as much fun playing “Dungeons & Dragons” now that they’ve faced off with real monsters?

Variety spoke with the Duffer Brothers about bringing the first season to a close, how they came up with the look of the Upside Down and its hungry inhabitant, the fate of poor Barb (Shannon Purser), and what they’ve got up their sleeves for Season 2 (which Netflix has not officially greenlit… yet).

When you’ve got eight episodes to tell your story, how do you decide to pace out a reveal like the extent of Eleven’s powers?

Matt: The idea was to slowly tease her powers out over the season. In the original script we sold to Netflix, she was hunted by the agents, she exploded a door off its hinges [in the first episode]. It was pretty extreme right away. As soon as we started mapping out the season and realized we had eight hours, we started to scale it back.

A lot of the drive of the show is not just looking for Will, it’s learning about her and her backstory, and how she connects to all of this. We built an entire backstory for her, and the trick was where to drop those puzzle pieces in. By the end of episode six, you know a lot about her. It was the same approach we had with the monster, that sort of “Jaws” approach — hint at it but don’t show much, so you have somewhere to go.

Ross: When you finally do reveal it, it has more impact. With Eleven, when you see her do these extreme things later on in the season it has impact. You start simply — with floating a Millennium Falcon, very small things, and hopefully it builds and builds.

How much do we know about Eleven’s true origins at this point, and how much did you want to keep a mystery?

Ross: We get the hint that her mom was involved in the experimentations back in the day resulting in her being born with these powers, but what we wanted to do with the show — and this season specifically — was mostly seeing the mystery and these extraordinary things through the eyes of these ordinary characters. By the end of the show they don’t know or understand everything. That is purposeful.

We do cut away to the government occasionally for these pops of mystery or horror, but what we didn’t want was to have a scene of the scientist just sitting down to explain everything. We wanted to slowly peel back layers of this mystery for audiences through the eyes of these very ordinary people. It’s not all solved by the end of the season. We wanted to resolve the main mystery of Will being gone, that was the story of this season.

Do you see the government or science conspiracy angle as a long-term mystery for the show?

Ross: There’s a lot there we don’t know or understand. Even with the Upside Down, we have a 30-page document that is pretty intricate in terms of what it all means, and where this monster actually came from, and why aren’t there more monsters — we have all this stuff that we just didn’t have time for, or we didn’t feel like we needed to get into in season one, because of the main tension of Will. We have that whole other world that we haven’t fully explored in this season, and that was very purposeful.

Matt: We wanted a simple drive and a somewhat simple mystery with bizarre pops of supernatural horror and then add a larger mythology behind this rift that we only know and refer to as the Upside Down because that’s what the boys decide to call it. Everything they’ve learned about it is kind of hypothetical. They’re theorizing based on their knowledge from fantasy gaming and their science teacher, Mr. Clarke. That’s as much as we get to understand it. I think part of it is us thinking in terms of horror, it’s scarier when you don’t fully understand what’s happening. If you were to encounter something from another world or dimension, it would be beyond comprehension. We talked a lot about Clive Barker and his stories. They’re very weird, and the weirder it is, the more inexplicable it is, the scarier it is.

As you head into future seasons, have you thought about how much of that 30-page document you want to reveal and explore?

Ross: We leave these dangling threads at the end. If people respond to this show and we get to continue this story — we had those initial discussions of where we might go with it. If there was going to be a season two, we would reveal more of that 30 page document, but we’d still want to keep it from the point of view of our original characters.

Even though you tell a complete story within the season, you end on a couple of major cliffhangers — the first being Eleven’s disappearance. Did you want to hint at where she’s been with the scene of Hopper leaving Eggos in the woods?

Matt: Obviously something happened to her when she destroyed and killed that monster and we don’t know what she went. Hopper is left with this guilt because he sold her out. We wanted to leave it sort of mysterious exactly what he knows… Have there been sightings in the woods or is he hoping she’s out there or has he already made contact with her? We don’t answer any of that, but we like the idea of potentially putting her and Hopper together.

It also seems that the Upside Down has changed Will or maybe he’s brought some of it back with him. What can you say about the flash he has in the bathroom?

Ross: We love the idea that [the Upside Down] is an environment that is not a great place for a human being to be living in. Will’s been there for an entire week, and it’s had some kind of effect on him, both emotionally and perhaps physically. The idea is he’s escaped this nightmare place, but has he really? That’s a place we wanted to go and potentially explore in season two. What effect does living in there for a week have on him? And what has been done to him? It’s not good, obviously.

Poor Barb. I was a little surprised she actually died. How did you decide someone would have to go?

Matt: I love that you said “poor Barb,” because that’s the go-to phrase we use. With the first episode we wanted someone to die very quickly — which was the Benny character [the diner cook played by Chris Sullivan] — someone set up who looks like a substantial character and dies. And then Barb who looks like a substantial character. We wanted it to feel unsafe.

One reason we fell in love with television is we’ve seen so many movies and they tend to follow a very similar pattern. Television has been breaking narrative rules. George R.R. Martin obviously pushed that to another level [with “Game of Thrones”], you suddenly don’t feel safe and it freaks you out. Every scene has a little more tension in it.

It’s something we want to preserve as we go into season two, where you feel everyone including the kids is unsafe and anything can happen. We pushed it this season with Barb, but I want to continue to amp up that threat. It makes it scarier, but it’s also sad. Shannon Purser who played Barb, we fell in love with her. She had never acted before, this was her first role in anything, she blew us and everyone away. It was sad to lose her, but some people have to go.

I also didn’t expect to see Nancy reconcile with Steve at the end. He crossed a line at a certain point in the season but then you pulled him back in. Was that by design?

Ross: That was not the original plan. A lot of credit goes to Joe Keery [who plays Steve] because he was much more likable and charming than we originally had envisioned. If you read the pilot, he’s the biggest douchebag on the planet. It’s not that he’s a flawless character, he’s flawed, but Joe was so good we started to fall in love with the idea that he has an arc himself. He’s maybe not the perfect guy, but he’s maybe in with the wrong crowd. As opposed to him turning Nancy to his side, maybe it’s more Nancy turning him to her side. We liked giving him that arc.

We made that month-long jump at the end of the season, and we don’t exactly know what happened in between. We feel like we hinted at it in the hospital scene where we’re seeing Jonathan with his brother, and how happy he is, whereas Nancy obviously still has this tremendous guilt and anger over what happened to Barb. She’s lost someone, so even though there is a victory, it’s certainly bittersweet for Nancy and she needs someone in that time.

Matt: The other thing that appealed to us about the idea is it felt like in a movie world she winds up with Jonathan, who is seemingly the nicer kinder gentler guy. But it felt almost more real to us that she would wind up back with Steve, this heartthrob who she’s had a crush on for a long time. It’s surprising, but it felt more honest.

That’s the fun of television. You can find an actor and they’ll inspire you to change and evolve a character in a way you hadn’t initially planned on. It was the same with the character of Dustin. Dustin was just the nerd, and you find these kids and they surprise you — who they are as people, who they are as actors. They inspire you to write something better for them. That’s what’s fun about television. You don’t have the opportunity to do that as much in film.

How did you come up with the design of the monster?

Matt: It’s been a lifelong dream of ours to build a monster and put him on screen. It was very important early on that we actually create it. And that we have a person in a suit with animatronic elements so we can film it in real time with our actors. We grew up on genre films before computer graphics, so the movies that scared us the most, the horror elements were done practically. We wanted to get back that.

We had worked with a concept artist before — Aaron Sims — and he’s absolutely incredible. We talked a lot with Aaron about what we wanted this monster to look like. We talked about H.R. Giger, Guillermo del Toro, Clive Barker — we tried to find what about their monsters was so effective to us, they tended to be humanoid but there was something very bizarre about them. If you were going to encounter something from another dimension it would be very weird. We started to play around with it, and we ended up with the guy you see on the show.

Fortunately, Spectral Motion is another company that built a lot of monsters for Guillermo. They built our monster and they’re really great. They have an amazing robotic engineer who built the head — we call them petals, the pieces that open up when it needs to feed. Those were completely animatronic, programmed in a way that they moved in random patterns, which made them feel very organic and alive. There were times we couldn’t do what we needed to do with a guy in a suit and then we turned to computer graphics. Fortunately, Aaron also had a vfx company. He cared so much, the monster was his baby. He and his team did a really great job bringing it to life on a really tight television schedule.

Ross: The actors loved it. Sometimes you see a monster sitting down in a chair and it looks ridiculous. The toddlers on set were terrified of it. Millie, who plays Eleven, has a sister who was about three, and we had the twins who play Holly Wheeler. It was freaking them out to a major degree. Someone went up and told them that the monster was not a bad monster, it came from Monsteropolis from “Monsters, Inc.” After that it was all good.

And how about the look of the Upside Down?

Ross: We wanted it to be like a dark shadow of our world. We knew it was something we could use practical things — we could build off of our sets and locations — it’s that mix of practical and visual effects. A lot of those vines and the growth we did build — a lot of them were moving and pulsing. When you do something like a full-on city street, visual effects has to take over at some point. And a lot of those practical effects were added as well. It was about, “What would our world look like if this was an evil shadow of it?”

Matt: We talked about “Silent Hill,” the video games were an inspiration, and “Alien” was an inspiration, in terms of the look. When we initially pitched the show we said, “We’re never gonna go into this alternate dimension.” We just didn’t think we could pull it off. I think we were imagining shooting our characters in front of a green screen and it would look awful. We came up with a design concept we knew we could effectively pull off. It was important to us that we never tried something we wouldn’t be able to do, or would look cheesy.

Ross: The limitations can be helpful. We couldn’t build a full-on CG alternate world. There’s no way we could’ve afforded or had time to do it. In a lot of ways to us it felt practical because everything is there. It all feels more real because it is real.

Was there an inspiration for Eleven’s look and conception?

Matt: That came very quickly. For some reason we decided she was going to emerge from the woods in this dirty hospital gown and her hair was going to be buzzed. The trick was no little girls wanted to buzz their hair, it’s not something either them or their parents are into. Even with Millie there was a lot of talk about it. They were hesitant because it was like, “Is this gonna cost her roles?”

Fortunately, it was the time that the “Mad Max: Fury Road” marketing was really ramping up. We showed Millie a picture of Charlize [Theron] as Furiosa and said, “Does she not look badass?” She was like, “Yeah.” We said, “You’re gonna look badass, exactly like Charlize.” She’s like, “OK, let’s do it.” Thank God for “Mad Max.” We shaved it off within 10 minutes. Her dad ran out crying, but within a week her entire family all loved it.

Ross: We needed something that would make her not feel like she fit in in this world. She’s growing her hair now, but it’s still very short. It’s taking a while.

Eleven kills a lot of people in the show. Was there any hesitancy about that?

Matt: It was important to us. I think when the show started out our vision for it was darker than what it wound up being. “Really dark Amblin” was our original pitch. We wanted to push things further than say “E.T.,” so it didn’t feel completely safe.

Ross: We wanted to keep that danger with the kids. It’s not just a fun girl with superpowers. This is someone that if she wanted, or even accidentally, could seriously hurt one of them. She’s a bit of a wild card. She doesn’t fully have control. When they get in the big argument and she telepathically flings Lucas across the junkyard, if he had landed poorly he could’ve been paralyzed.

Matt: We always wanted to keep the stakes high. When you’re looking back at “Stand By Me,” the stakes feel very real. The kids never feel completely safe, even though there is an element of fun and you love those boys. There’s this consistent danger with Kiefer Sutherland coming after them, the train coming so close to them — we wanted to always keep the kids in real danger, that’s not “Dungeons & Dragons” danger.

Ross: I think it’s also the Stephen King thing, he doesn’t mind killing people. He has children die. It’s important to us that even though it’s a show with four kids in lead roles that it wasn’t a show designed specifically for kids. I like the idea that younger audiences can watch the show. But when I was young, I wanted to watch stuff that was meant for someone older than me because it seemed so much cooler. I like the idea that if kids are watching it doesn’t feel like it’s targeted at them, it’s pushing things in a darker, more dangerous, scarier direction than maybe they’re comfortable with. That’s exciting.