After he finished, the future director of 2016’s horror film Hush and the same year’s Ouija: Origin of Evil had two thoughts. The first? That it was “one of the most brilliant books” he had ever read. And the second…?

“That it was completely unfilmable,” laughs Flanagan.

Two decades on, Netflix viewers will be able to make up their own minds as to whether the young Flanagan was right or wrong when the streaming network debuts the director’s adaptation of King’s book on Sept. 29. But one audience member Flanagan doesn’t have to worry about is the novelist himself, who has already given his thumbs-up to the film, which stars Carla Gugino (the Spy Kids films, Watchmen) and Bruce Greenwood (Star Trek, Thirteen Days).

“One of the most nerve-wracking moments of my career was sending the first cut off for him to review,” says Flanagan. “You know, he’s got strong opinions about how his work is adapted. It’s no secret how he feels about how Stanley Kubrick treated The Shining. He came back very quickly and really loved the film. He sent me an email that — I’m not exaggerating — I actually printed, and framed, and have hung in my living room. That was one of the most gratifying moments of my career.”

King’s novel concerns a married couple, Jessie and Gerald, and a sex game in a secluded cabin which goes awry, leaving Gerald dead and Jessie handcuffed to a bed, with no apparent means of escape.

“The biggest obstacle, when it comes to an adaptation for this, is that the protagonist is handcuffed to a bed, alone in a room, for the duration of the novel,” says Flanagan, who co-wrote the script for Gerald’s Game with his regular collaborator, Jeff Howard. “Everything that happens is happening entirely in her head, and in her memory, and it’s very hard to present that in visually compelling way.”

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Not that this prevented the idea of adapting the book from becoming something of an obsession for Flanagan, whose other credits include 2011’s Absentia and 2013’s Karen Gillan-starring haunted mirror tale, Oculus.

“When I moved to Los Angeles I would carry around a hard copy of Gerald’s Game in my bag, when I would take general meetings, in case anybody ever asked what my dream project was,” he says. “Then [I] would always pull it out, and they were either not familiar with the material or, if they were familiar with it, they would say, ‘Well, that’s unfilmable!’ And then Stephen saw Oculus, and really liked it, gave me a chance to whip together a screenplay. I started working on it after Oculus, and finished the script, and he loved the script, and then for years it just kind of stalled. We couldn’t find people that could wrap their head around the movie that we wanted to make.”

That situation changed after Netflix bought Flanagan’s Hush, about a deaf woman, played by Kate Siegel, who is tormented by John Gallagher Jr.’s psychopath at another remote abode.

“Hush came out, and we had this new relationship with Netflix, and that kicked the door open finally for the project,” says Flanagan.

The role of the film’s heroine, Jessie, would seem like a plum one for an actress, but Flanagan reveals it was a difficult one to cast.

“Jessie, as a character, we knew from the beginning was either going to be irresistible to the right actress or it was going to scare them off,” says the director. “Finding an actor who was brave enough to take on not only the level of exposure and vulnerability that the part requires, but also carry the majority of this movie, and you’re not even really going to be able to move for most of it, that was really, really hard. We were fortunate to get Carla, who, when we had first started the process, was unavailable for us, and had another project kind of change everything around, and it popped up very, very late in the process that she was available. And she really took ownership of the movie and of the character. I’m admittedly biased, but I remember in my first conversation with her that we both felt like this could potentially be the performance of her career. And I think it is.”

Flanagan admits that making a film set for the most part in a single room was a good news-bad news situation.

“It’s nice because you don’t have to travel too far from the hotel,” says the director. “But, yeah, it’s very much good news-bad news. It presents so many challenges, working in a confined space like that; it puts an enormous amount of pressure on us to keep the camera work very dynamic, and to resist any temptation to repeat ourselves. So much of it is in that space, we kind of needed to be floating through it like the Eye of God. And, for that, I was fortunate again, because I’ve been working with Michael Fimognari as my DP (Director of Photography) since Oculus, and so we’ve built quite a rhythm together, and I think that this is some of his finest work.”

So, how exactly did Flanagan turn King’s “unfilmable” book into, you know, a film?

“Well, it’s tough, I don’t want to spoil things,” he says. “The movie as we designed it really lives and dies on its two principal actors and the weight of the success of our adaptation was put squarely on Carla and Bruce. I got to play a little bit with some of the techniques that I had started to explore in Hush, which in a lot of ways was a dry run for this movie, and in Oculus as well — kind of blending timelines, and delving into visual experiences characters are having that may or may not be real. It’s been something that I’ve enjoyed playing with on a number of projects and this was kind of the culmination of a lot of those little tricks that I learned along the way. I’m glad it took as long as it did for this film to be made, because I think if I had tried to do it before I had made some of these other movies, I don’t think I would have been up to it. It was certainly the hardest one. The stars finally came together. It just took half my life for it to happen!”