The showrunner of "The Strain" discusses how the challenges of writing a series finale have changed since "Lost."

Carlton Cuse knows how to end a TV show. Before co-writing the last episode of Guillermo Del Toro and Chuck Hogan’s apocalyptic vampire story, “The Strain,” the showrunner and executive producer had already written four series finales. Four!

In 2000, Cuse penned “Final Conflict” Part 1 and Part 2 for the CBS action drama “Martial Law.” A year later, he wrote the ending to his breakthrough broadcast series, “Nash Bridges.” Then came the landmark finale of “Lost” in 2010 and, earlier this year, he dimmed the vacancy sign at the “Bates Motel” in a beautiful closing chapter.

Next up, as “The Strain” wraps up on Sunday, September 17, Cuse said he knew exactly how to end it.

“I think that to end a show, you have to really look at what your show is fundamentally about and then figure out your ending from that,” Cuse told IndieWire. “In the case of ‘The Strain,’ it’s a graphic novel epidemiological thriller. I think we view it as a wonderful popcorn movie experience: There is a clear force of antagonism, this Master — a vampiric parasitic creature — and there are a bunch of protagonists that are trying to do him in.

“I think that the fair and right ending to ‘The Strain’ is one that gives you a real sense of what the ultimate fate of these characters are and ultimately resolves the conflict between a clear force of antagonism and a clear force of protagonism.”

Not every show necessitates a showdown between good and evil. Some are more complicated, including one of the most hotly debated series finales ever.

“Certainly when Damon [Lindelof] and I wrote the ending for ‘Lost,’ we felt like there was no version where we could answer all of the mystery questions without feeling didactic and unsatisfying,” he said. “The attempt to answer those questions would ultimately just lead to more questions. What we felt was important was to provide a character resolution to explain what happened to them and to provide them with a sense of emotional closure.”

A lot has changed since “Lost” ended, including the very system that led to all those questions stacking up. Prestige drama projects have been snatched up by cable and streaming outlets, which allow for shorter seasons and less seasons overall. (Both length-related issues were regular sticking points for Cuse and Lindelof when negotiating with ABC.)

Such shifts in how television is made emphasize the importance of Cuse’s experience writing finales: He’s done it under the harshest conditions as well as the most idyllic.

Back when “Lost” was made, Cuse describes the network television mentality “like the pony express: you rode the horse until it dropped dead from underneath you… The idea that we made 24 hours in the first season, 23 the second, and 22 in the third, I mean it’s just… In an era of orders of eight [episodes per season], it’s kind of incomprehensible.”

“The Strain” has never had more than 13 episodes per season, and the final two years lasted just 10 hours. That’s a drastic shift compared to broadcast standards, and a big win for the writers.

“The imperatives of network television meant that we had to generate massive amounts of story material,” Cuse said about “Lost.” “In a show that’s a mystery day show [meaning mysteries were introduced almost every episode], that meant generating a lot of mysteries. So the yarn ball of mysteries by the time we were going into the final season was just massive. Innately, because of the format in which we were working — which was network television — we were constrained and there’s no way that that couldn’t affect the way the show ended — and certainly the mixed perception of it.”

Cuse credits supportive networks like FX for allowing more series to end the way they should, and trusting the writers to make bold choices is a big part of that. He regularly credits his collaborators when discussing any of his shows, and “The Strain” is no exception.

“I co-wrote the final episode with Chuck Hogan, so we developed it together,” Cuse said. “All of this was stuff that we ran by Guillermo, who had notes we incorporated. He had some specific ideas about the ending of the show that we embraced and I think really added some wonderful resonance and nuance.”

Those ideas, as well as Cuse and Hogan’s, were not beholden to the books which inspired the series. The ending to the TV show sounds like it will be very different from what’s on Hogan’s pages.

“We absolutely did not feel wedded to the books,” Cuse said. “I think when you adapt anything you have to let it become its own thing, and in the case of this trilogy of books, we always saw that there were sort of landmarks in the books that we would hope to get to, but we didn’t feel compelled to do anything in the books.”

“The books are one thing and this television show became something quite different,” he said. “While they share some of the same landmarks, the television show is its own story and we treated it as such. So the ending for the television show is something that felt appropriate from really just evaluating the narrative of our television show, not the books. We didn’t feel like we were kind of compelled to try to swing back around and end the television series the same way as the books. It was just sitting around deciding, ‘OK, what’s best for our show?'”

Come Sunday, viewers will discover Cuse’s answer.

“The Strain” series finale airs Sunday, September 17 at 10 p.m. on FX.

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