On Dec. 28, Netflix members got a year-end surprise: a new Black Mirror thumbnail on the streamer's homepage signaling that the rumored interactive offering from Charlie Brooker and Annabel Jones had finally arrived.

Black Mirror: Bandersnatch launched Netflix into the live-action interactive space with an immersive, choose your own adventure-style story after a secret, nearly two-year collaboration between the streaming giant and the Black Mirror duo of Brooker — the Emmy-winning series’ creator and writer — and executive producer Jones. Netflix dropped the film with little promotion, and the strategic marketing approach worked: Bandersnatch soon was trending worldwide on Twitter, and in less than 24 hours Reddit threads and fan sites had created their own flowcharts to track the multiple endings and complex story paths of the film's branching narrative — the result of trillions of unique permutations created by Brooker’s nontraditional script.

“We personally were just relieved,” Jones tells The Hollywood Reporter of the audience reception. “The big thing we wanted to achieve was earning the right to use the interactive layer. When you take on something of this enormity — and it’s not a linear experience — you don’t know how people are going to respond. We didn’t have control; people were left to make the choices they wanted. So we were deliriously excited that people were responding to it like it was a film. The fact that they cared so much about the protagonist Stefan [played by Fionn Whitehead], that they were striving to give him the happy ending they thought he deserved, made us feel like we created something that was having an emotional impact."

Brooker, who wrote Bandersnatch's 170-page script, says it was that collective desire to give Stefan a happy life that notched a win for the storytellers — even though viewers should know by now that uplifting endings are hard to come by in the Black Mirror universe.

"One thing we noticed was that people were disappointed they couldn’t get a happy ending for Stefan or a happy ending overall," Brooker tells THR. "The closest we get is the train ending, which is bittersweet and sort of melancholy. Some people were disappointed, to which I was thinking, 'Have you ever seen Black Mirror?' But, hopefully, that means we succeeded. That’s a testament to Finn’s performance that people cared that much."

He continues, "There was a concern that people might look at this as a game or approach it thinking it was a gimmick, but if people were being drawn into the story and into the character so much that they were then totally shattered that they couldn’t fix his life, that was evidence that they were absorbed and were buying it and were engaged. So that was gratifying, even though that was sometimes a criticism."

Three weeks after the release of Bandersnatch, which centers on a programmer (Whitehead) who realizes he isn't in control of his life when adapting a fantasy novel into an interactive game, Netflix shared some viewership data. Bandersnatch, directed by David Slade, filmed five hours of footage but thanks to first-of-its-kind “state-tracking” technology — which tracks viewers’ choices as they go and was developed by Brooker and Netflix’s product team — each viewer has a unique experience that can run anywhere from 90 minutes to two-and-a-half hours.

As a result, there was an "Easter-egg ending," and the train conclusion was confirmed by Netflix as the hardest to reach of the five main endings (there are variants on each ending). Netflix also shared the percentage breakdowns from three key choice points in the story (including the opening meme-worthy choice of breakfast cereal), but never released total viewership numbers.

“We talked about the relevant information that people care about, and in the case of Bandersnatch, you knew who was watching it. It was much more about how are you watching it?” Carla Engelbrecht, Netflix's director of product innovation, tells THR. “The number that I was waiting desperately to see and that I felt would be the determination of success or failure was: were viewers actively making choices? We saw in the data that 94 percent of viewers were actively making those choices. We’re talking about decades of training that when you watch TV, you put on what you want, drop that remote and snuggle into the couch. And instead of snuggling back, people were actually leaning in to make these choices."

She adds, "That was the one piece of data where I felt like, ‘Oh, we’ve got something here. We are in it.’ It was absolutely a success.”

Engelbrecht’s team hosted a summit of sorts with Brooker and Jones, where they debriefed the pair on the viewer trends that had been uncovered, including percentage breakdowns of choices, how many endings viewers reached and how many times they played it over. In follow-up research, Engelbrecht and her team watched Bandersnatch with members and discovered that viewers indeed had a "deep, deep empathy" for the protagonist Brooker and Jones had created. To Engelbrecht's surprise, she was also proven wrong in her prediction that 90 percent of the audience would pick the dire option to "Kill Dad" later in the story. "I am very glad that humanity is generally good and that it did not go the way I feared it might," she says with a laugh.

“They were very happy,” says Brooker of the Netflix response, which Engelbrecht says is reflected by the fact that interactive projects have continued to roll out from the streamer. “People stayed with it, probably longer than we anticipated," Brooker continues. "The whole thing was very experimental. There isn’t a right or wrong way. We deliberately play with you throughout the film by setting up very difficult choices, and there was a lot going on behind the scenes to make that happen. If we were tackling another interactive story, I now have a wealth of stuff in my head, not only from the process of doing it but also from the information you get back about the kind of choices people made.”

From a storytelling perspective, the biggest challenges were cohesion among the endings and consistency with the characters. "How do you cede control to the viewer while also maintaining characters that are consistent? That’s a big, big challenge narratively," says Brooker. "In Bandersnatch, Stefan is removed from the choices. So he starts resisting them and that means, regardless of what you thought of the film overall, that he is a consistent character because he becomes aware that he is someone struggling against this compulsion. No matter what it is that you’re compelling him to do, his reaction is always that of someone who is in resistance."

In order to maintain that throughline, not every idea from mastermind Brooker made it into the final edit. A central puzzle had to be trimmed down (the result is a call to action where viewers make a phone call) and Brooker's pitch to insert a 90-minute film within Bandersnatch was also nixed ("Knowing Charlie, I will not be surprised if that finds a way into something else," says Engelbrecht). Any time an edit was made, much like a Jenga puzzle, the structure of the story was impacted. As a result, there is one hidden scene that's impossible to reach, but that Brooker and his team kept in the final piece anyway.

"If you’re doing a linear story and there’s a scene you want to remove, you can get around that relatively easy by joining the plots some other way. When it’s something that’s like a four-dimensional flowchart with logic flowing through it, you can cause all sorts of problems," says Brooker of the bizarre technical challenges that arose. "There were points where we could not predict where you would end up. It was sort of out of our control."

He continues, "There is a scene that you can't get to but was too late to be removed. It's the third time Colin [played Will Poulter] encounters Stefan in the office and he’s almost finishing his sentences. Somebody managed to find this. I think they illegally ripped it and put it on YouTube. But the doors are locked in terms of you getting there. I remember saying in the edit that if you can’t get to it, we should just dub the Beatles all over it and not have to pay because no one will ever know. It’s a good thing we didn’t do that!"

Brooker and Jones like to tackle a new genre with every episode of their techno-paranoia series and, in the end, Bandersnatch notched them two firsts. "When we looked at the film, we were beginning to see that it was like a whole Black Mirror season in one," says Jones. "Some of the paths are quite comic and some are horrific; we’ve got a fourth wall-breaking meta sort of fiction going on. But they all got to that central conceit of Stefan becoming aware that he’s being controlled by this thing, which is you at home. In that sense, it does feel like one season of Black Mirror all with one protagonist, which is something we haven’t done before."

For Netflix, the most useful genre element to come out of the creative process with Brooker was perhaps the streamer's new story-mapping tool. After realizing there was no tech to create a branching-narrative script, Brooker worked with Engelbrecht’s team to develop an in-house program called Branch Manager. Netflix is now applying the tool to all of its interactive projects, including Bear Grylls' You vs. Wild (released after Bandersnatch), current interactive children’s programs on the streamer and the upcoming Unbreakable Kimmy Schmidt interactive special. Some advice for Tina Fey when breaking into interactive comedy? “A lot of fourth wall breaking,” says Jones. “Then you can just have so much more fun and have it be more liberating.”

Bandersnatch's meta fourth wall-breaking path was originally tucked away, and only later made easier to reach because it was "so irresistible" to the storytellers. Brooker and Jones are still split on whether that was the right decision. "Not by a huge margin, but more people went down the comic 'Tell him he’s watching Netflix' path the first time around than the more dramatic, serious path and that means those people experienced it in a different way. People who recoil at the meta nature, I think, would then be more likely to be dismissive of the whole thing," says Brooker. Jones counters: "But in order to get to the more serious train ending, Stefan, just like any character, had to go through experiences. He had to learn various things before reconnecting with his past. In that sense, we used the form to not only tell the story but to support the story."

As the first interactive comedy, the team behind Kimmy Schmidt will have to pave the way when it comes to setting up jokes that pay off in non-linear storytelling. In addition to exploring more genres (horror and romance are mentioned), Engelbrecht says she hopes to see projects expand the number of choice points and even go entirely interactive. "Different from the model of it being an episode within a show, what if the entire show is interactive?" she says. "We’re overjoyed with the interest, both from our members as well as the creative community. I do feel like there’s a world of opportunities for us to explore."

Ultimately, hindsight is 20/20 and, though the Black Mirror pair aren't privy to the total viewership number, the retention levels in the data show how open people are to immersive stories. "When people came to Bandersnatch, nearly everyone stayed with it," says Jones. "There was not a sharp switch off when people saw the first choice point come up and that’s really interesting for the future of this form."

Bandersnatch was originally part of the forthcoming fifth season of Black Mirror, which will launch on June 5, and one of the three new stories had already been shot when they embarked on their interactive journey. "It was all in conjunction," says Brooker of working on the linear and non-linear stories. "The challenges that come with doing a linear story are more familiar. It’s a relief in some ways to be doing a linear edit where you’re not having to worry about if you’re creating a logic bomb if you remove something. That was why we ended up having Bandersnatch as a stand-alone thing — it was basically the length of an entire season and in one go."

When speaking to THR shortly before launch, Brooker and Jones — maybe half-jokingly — doubted they would have done the project if they knew the level of complexity involved. Now that the dust has settled, the pair are much more open to returning to interactive storytelling.

“While we were making it we kept saying, ‘We’ll never do this again.’ And then I guess, perhaps like childbirth, you block out the pain and approach it a second time!” says Brooker with a laugh. “By the end I was saying, ‘Oh I’ve got an idea.’ It’s definitely something I would do again and I think there are lots of ways to tackle it. The biggest challenge is narratively. It would have to justify being interactive otherwise, why put yourself through that?”

A version of this story first appears in a June stand-alone issue of The Hollywood Reporter magazine. To receive the magazine, click here to subscribe.