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Who’s the killer in ‘The Undoing’? The novelist behind the series wonders, too.

As the novelist behind “You Should Have Known,” the source material for HBO’s “The Undoing,” Jean Hanff Korelitz has come across plenty of viewers dying for her to spill the beans on the limited series’ murder mystery.

Luckily for Korelitz, she has a foolproof defense for spreading spoilers from the series: She doesn’t know who the killer is, either.

Series producer and writer David E. Kelley “very kindly showed me the scripts, except for the all-important episode six, which nobody has seen,” Korelitz, 59, says of the show, which airs its fourth episode Sunday. “People keep asking me, ‘Who did it?’ And I’m like, ‘I have no idea!’ I know who did it in the book, but I’m prepared to be very surprised along with everybody else.”

Published in 2014, Korelitz’s “You Should Have Known” is a simmering character study about a New York therapist and author named Grace, whose new book cautions women to recognize the flaws in their romantic partners. But when her husband, Jonathan, a respected pediatric oncologist, goes missing after the murder of a local mother, Grace is left to process the blind spots in her own marriage and rebuild her life with her young son.

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When Kelley acquired the rights to “You Should Have Known,” casting his “Big Little Lies” collaborator Nicole Kidman as Grace, he told Korelitz that he planned to use the premise and characters as a launchpad for a twisty whodunit. As part of that dramatic reimagining, Jonathan — who is only referred to in the book and never appears as an active character — became a co-lead played by Hugh Grant. And Kelley made it clear that wasn’t the only change he’d make.

Earlier this week, Korelitz spoke by phone about “The Undoing” — and what it’s like seeing Kidman and Grant as characters who aren’t quite the ones she envisioned.

(This interview has been edited for length and clarity.)

Q: It’s no secret that “The Undoing” is structurally different from “You Should Have Known.” What kind of role did you have in the adaptation?

A: Zero! [Laughs] David E. Kelley was absolutely lovely, kind, charming, very complimentary about the book, but it was clear. He said, “I’m going in a different direction.” And I said, “Fine.” I mean, what am I going to do — question David E. Kelley? That’s like saying to Picasso, “No, I don’t like that color blue.” I have great faith in him. I’ve been watching his work for decades and I can’t think of anybody better.

Q: And how have you responded to the changes?

A: I made my peace with it a long, long time ago. They said, “We’re casting Hugh Grant,” and you don’t cast Hugh Grant to play a character who’s not there. It was obvious. I think David even said to me, “The Jonathan character is going to be front and center.” And I thought, “Great!” My book is not that interested in him. I mean, we’re interested in Jonathan as a sociopath and as the instigating factor in all of this, but the book is about Grace.

Q: What do you think of Kidman’s take on Grace?

A: I think she’s a great actor, so I’m incredibly impressed with her. I mean, there are other huge differences. In the novel, everybody’s Jewish, so all of these people have been re-ethnicized, and that is, of course, another variation of the story. In the novel, [Grace and Jonathan] are not nearly this rich. A whole part of the plot is how poor Grace feels despite her obvious advantages. But [these details] are not crucial to the story, and now a different story is being told.

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Q: Another one of your books, “Admission,” was adapted into a 2013 film starring Tina Fey and Paul Rudd. How has you experience with “The Undoing” compared to that project?

A: Well, it was quite different. There was just a very pro-writer vibe on the “Admission” set. And people kept telling me, “This is not normal for the writer to be welcomed onto the set and allowed to hang around.” I gather that [“The Undoing”] was a much more classic thing. For many of the people that I met on the “Undoing” set, who I was introduced to as the author of the novel, the biggest response was, “Oh, there’s a novel?” But that’s okay — it’s just a different animal.

Q:You Should Have Known” came out six years ago. What has it been like seeing this sudden surge in interest tied to the HBO series?

A: I’ve never had an experience like this where people are wanting to read my work, and that’s incredibly gratifying. One thing I worried about — and which seems to be happening, although I’m trying to not be upset about it — is the fact that they’re comparing it to the TV show. Of course, why wouldn’t they? But I see a lot of, “It’s not as suspenseful as the TV show. It’s all about [Grace] and what she cares about and all these details.” And yeah, that’s the book I wrote! We were here first. I think the book is suspenseful, and there are plenty of plot twists in it, but you do have to wait for them because the book wants to know what that life-altering experience for Grace is like. That is its raison d’etre.

Thomas Floyd is a multiplatform editor who writes about arts and entertainment for The Washington Post.

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