When Luke Jennings’ 2018 novel “Killing Eve: Codename Villanelle” was optioned for a TV series, the British writer was given a list of potential scriptwriters to adapt his story about a witty, glamorous assassin who becomes obsessed with the MI-5 agent assigned to take her out. One name jumped out at him: Phoebe Waller-Bridge.

“I saw her [play] ‘Fleabag’ in the Edinburgh Festival. It was so great and I thought it would be appropriate if there was a woman as a showrunner,” says Jennings, 66. “I sort of ambushed her after the show and we spent the rest of the day talking. I very much trusted her instinct about the characters.”

Waller-Bridge, of course, went on to great success with BBC America’s “Killing Eve,” collecting nominations and awards on both sides of the Atlantic, and series stars Sandra Oh and Jodie Comer were both nominated for Emmys last week.

But Jennings was involved in the series behind the scenes in the writers room, on the set and in the casting process.

Unlike Jennings’ books, which early on describe Villanelle’s (Comer) harsh upbringing in Russia — and her rigorous training for the assassination business — Jennings and Waller-Bridge agreed that “there would be no expository conversations. You would just have to figure out her back story as you went along and piece it together.”

Finding the right actor to play Eve Polastri, Villanelle’s nemesis, was an extensive process. The BBC wanted an American actress for the role. Some, Jennings says, were too fancy to play the bedraggled M1-5 agent. “We saw quite a lot of American actresses who would have been great on ‘The Good Wife’ or whatever,” he says. “But if you’re going to have someone with a North American accent in a [drab] London office, you have to believe that Eve’s been going to work on the bus. She has the wardrobe of somebody who can’t be bothered to iron. She’s a junior civil servant and her husband’s a teacher. They are both hard jobs and poorly rewarded.”

Enter Oh. The “Grey’s Anatomy” star was “perfect,” says Jennings: “She has no vanity and she’s funny.”

‘You have to believe that Eve’s been going to work on the bus. She has the wardrobe of somebody who can’t be bothered to iron.’

With production underway, Jennings made himself scarce. He had to start working on the second volume of the “Killing Eve” book series, “No Tomorrow,” which was published this year in Great Britain. “There comes a point where everyone’s got a job to do. You don’t want to lurk around,” he says. “I wanted to sort of talk to Jodie and Sandra every so often.”

The success of the series has meant the publication next year of a third volume in the “Killing Eve” series, called “End Game,” which Jennings says “will probably be the last.” Unlike “Game of Thrones” fans, who regard the George R.R. Martin books as sacred texts, “Killing Eve” readers have been open-minded about discrepancies between the text and the TV show. They don’t prefer one to the other.

“The books and the film mesh together into a universe of characters and possibilities,” Jennings says. “The fans have parsed the books very closely, line by line. They want to know: What was Eve’s maiden name? What is Villanelle’s star sign? I get contacted by them all the time. It all means a great deal to me. The show and books have also struck a chord with LGBT audiences, particularly young women, because Villanelle takes her life into her own hands. That they like a lot.”

The success of “Killing Eve,” which has been renewed for a third season, catapulted Waller-Bridge into the forefront of in-demand writers. It also exposed Jennings to an unfamiliar world, which has reaped him much acclaim but, at the same time, removed him from the decision-making process.

“You win some, you lose some,” he says. “There are decisions I wouldn’t have made. I’ve been a journalist and writing novels for 30 years. It’s a solo job. The moment it goes into TV it’s large numbers of people. It’s always going to leave your grasp. You can be cool about that. Or you can be distressed about it. There’s so much that’s great about the show. I think it would be churlish to carp about the details I don’t like.”

Warning: Video contains spoilers from the season finale of “Killing Eve.”