"Even if they aren't the final chapter titles, everything in that teaser is major," says Matt Duffer. "Some of the fan theories online are amazing."

A few weeks before heading back to Atlanta for the second season of Stranger Things, the Netflix breakout that made them TV's most in-demand siblings, Matt and Ross Duffer are still holed up in the Lake Hollywood rental they've used as a writers room all summer.

Contrary to the timing of the renewal, only made official a month prior to this late-September morning, the next batch of episodes seem set in stone. A peek at the discreetly shielded dry-erase board reveals thorough outlines for the nine-episode return — the same nine the Duffers teased out with chapter titles in a promotional video — and the writer-directors say they're nearly ready to power down their laptops and focus on casting, preproduction and the fall shoot. "I’m sure there is a more efficient way to take advantage of the fact that there are two of us, but we’re basically pretty dysfunctional apart," says Matt — who, along with his brother, made The Hollywood Reporter's annual rundown of the 50 most powerful showrunners.

Speaking with THR, the duo discuss the secrecy around their renewal, the surprising accuracy of Reddit theories that have popped up online and why those season two episode titles they recently teased may not be the same ones that premiere on Netflix in 2017.

Were you instructed to be coy about the renewal when you were promoting the first season?

Matt: They greenlit the second season before we even premiered. So when everyone was going, "Is there going to be a second season?" we had been writing the entire time. So we’ve been working on this almost all summer.

Ross: It drove me crazy. "Please, can't we just tell everyone we’re doing a second season?"

Matt: Netflix has its mysterious ways, but it actually ended up working because it had built up to this fever pitch. I guess that’s what they were intending to do all the time.

Did you have a good idea of what you wanted to do when you started?

Matt: Even way back when we pitched to Netflix the first season, we talked about where it would potentially go. They understood the potential. Season one does almost feel like a big movie that comes to a sort of ending. A lot of that was based on the character of Will and the repercussions of him being in this upside down world for a week. Exploring that would be the second season. Once we got into the room for season two, we started expanding our mythology. We never got boxed in, because we’re dealing with an alternate dimension. It feels like the possibilities are limitless. We’re building up the mythology in a way that we know now where we want the story and these characters to end. We have more of a game plan now than we did two years ago.

How have you been digesting the sudden fame of your young cast?

Ross: Well, they love it.

Matt: We’ve seen them a couple of times and they seem really good. They seem excited. No one was expecting this. But as they've gone on this whirlwind of a press tour, we’ve been huddling with writing.

Ross: It's been surreal in terms of paparazzi taking pictures of them. I can’t imagine that as a 12 year old. But they’re good kids, and they seem to be handling it very well.

Matt: People always ask, "What's it like working with four kids?" It was amazing. Sometimes their attention is all over the place, so it was just getting them to focus. We're on a TV schedule. You can't do 10 takes.

The announcement for season two revealed episode titles. Whose idea was that — and how nervous were you about committing to titles before filming?

Ross: That was all our idea, so if it blows up in our faces …

Matt: Netflix had another teaser, but it was about going back to stuff that had happened already. I thought it wasn’t exciting enough, and we wanted to provide some hint of where we were going in season two without giving anything away. I do think some of the titles will change. There were titles we didn’t want to put on there because we felt like it would give too much away. The whole season was already broken when we did this.

Ross: So we did have a lot that we could tease.

Matt: Even if they aren't the final chapter titles, everything in that teaser is major. But they’re ambiguous enough that no one is going to be able to figure it out. Some of the fan theories online are amazing. Most are wrong, but I’ve read a few that are right or very close. Is it Reddit? Some of those people have figured stuff out based off of the chapter titles.

Ross: Some of these theories are elaborate and smart. They're not crap. I love reading this stuff.

Matt: But we don’t go on Reddit.

Ross: No, people send us the stuff. I know I would never get out if I went in there.

How were you engaging online as more and more people watched?

Ross: We search Twitter, even though we’re not on it. Waking up that Friday morning, because the season was released at midnight, and seeing how many people finished the show and were already tweeting about it was crazy. It's sort of snowballed from there.

Matt: It’s really gratifying because, while you're making it, you're sometimes only getting feedback from two or three people. It was Shawn Levy and Dan Cohen, our two producers, and that was it. It felt really small, so it was insane to see it go out into the world and be experienced by that many people. We did one movie before that literally nobody saw. This has been a polar opposite experience for us. We’re trying not let it change our process. The good thing is that we have most of the story broken out before anyone started tweeting or writing about the show. Of course, a lot of what people want is already in the second season.

Where are you guys with casting?

Matt: We did a wide, wide search for one new kid that we're adding. She's a cool new character.

Nine is an even more specific episode count than eight. How did you land there?

Matt: It’s a weird number, right?

Ross: Netflix didn’t pressure us to do more or anything. We don’t feel like we need a full 10 episodes. But it can’t quite fit in eight, so that’s how we ended up in nine. And I think in future seasons it will be the same thing. Let’s break out the story and see what we need. Whatever number it ends up at, great. The thing I like about keeping it down to eight or nine is that we’re able to more meticulously control things and try to keep it from turning into this machine that’s grinding things out. If it gets much longer, it could become unwieldy and just turn into a traditional show — which is what we’re avoiding.

Matt: I think a lot of that has to do with the fact that we didn’t grow up watching any television. Mostly, we watched movies. We like the pacing of film, so what we want to as much as possible is make the show feel like a movie.

Ross: And what we’re actually able to do the second season is ramp it up like a coaster that just keeps getting bigger and bigger, and it builds to this big, climatic finale. That was harder in the first season because Will was taken right away and everyone was on high alert. The concern, if we did 13 episodes or something, would be having to tread water. We like this format, but we’ll see where the story goes.

Matt: And each episode can be however long we want it to be. I asked Netflix, straight up, "How many episodes do you want?" They really did say, "Whatever you think you need to tell the story" — even after the show became successful.