In "Big Little Lies," Nicole Kidman's last series from David E. Kelley for HBO, everybody was lying, and everyone knew it. But in their new project together, "The Undoing," it's not clear who's telling lies.

In the six-part limited series that premiered Sunday, Kidman plays Grace Fraser, a high-priced Manhattan therapist, married to Hugh Grant's Jonathan, a successful child oncologist. But when the beautiful mother of one of his former patients — and schoolmate of their young son, Henry (Noah Jupe) — is found bludgeoned to death, their lives unravel as unseemly pieces of the story begin to rear their ugly heads, in true Kelley style, with episodic cliffhangers that will leave jaws agape weekly.

"The cliffhangers are beyond genius," Grant tells The Washington Post. "The way I can really judge is that, when I was reading the scripts, did I want to quickly pick up the next one? And the answer was always 'Yes.' And that's very rare." It is Grant's character who leaves the audience — and his on-screen wife — wondering if he can be trusted at his word about his role in the events.

Kelley began developing the series a number of years ago, based on Jean Hanff Korelitz's 2014 novel, "You Should Have Known." After finishing "Big Little Lies," he shared drafts of the first two "Undoing" episodes with Kidman, who had a similar response to Grant's.

"He would slowly give them to me, so I was on the roller-coaster journey of it, not knowing what was going to happen when I would read the next," she says from Sydney, on the set of yet another upcoming Kelley series, "Nine Perfect Strangers." "I've been so fortunate to have him. He can write for me in a way that I've never experienced with a writer before."

Not long after signing on both as an actor and executive producer, Kidman and her producing partner, Per Saari, approached Danish director Susanne Bier to helm the series.

When Bier read the script for the first episode, "it could either have gone more thriller or more drama," she says, convincing Kelley to go with the former. Kidman agreed with the choice: "It could have been just a psychological study, without that pulsating 'Whodunit?' through it. And, like David, she loves taking people on a ride."

Though a few other actors' names had been tossed around, when it came time to cast Jonathan, Bier quickly suggested Grant. While the actor built his career long ago playing the charming, handsome, likable "Hugh Grant character," he was loath to continue that sort of typecasting, something he has quite purposefully stepped away from in recent years.

"I've been doing nothing but dark characters for years now," he says. "In fact, there was some hesitancy on my part. I could sense that they were thinking, 'Well, who better to convince everyone that Jonathan's a lovely guy than the old Hugh Grant persona?' I was resistant to go back to that."

He even worked up an entirely different approach to the character, giving him a backstory in Paris as a pretentious pseudointellectual. "I had the whole costumes and hairdo down and everything. But then I realized that what they were thinking was perfect for the coup d'etat that occurs shortly in" the series, he says.

In fact, the actor skillfully plays with that well-known persona to the benefit of the story. "Jonathan's always been very, very good at charming people," Grant explains. "And the debate you want the audience to have is, 'Is this entirely real, or is this studied and manipulative in some way?' And the trick with that is to try not to make him boring or nauseating, but ... to make people think, 'Is this guy a little too good to be true?' I wanted people to wonder if there was a little nylon in the cotton of my shirts."

Grace's main focus, Kidman says, is protecting her son, Henry, played superbly by young Jupe, a veteran of Bier's "Night Manager" and able to play emotional scenes at the same level as his co-stars. "He's just so fluid and brilliant at emotionally being able to access anything," Kidman says. "He's got the skills and depth of an adult. He's this boy-man."

Accessing those emotions is not quite so easy for Grace, a private introvert.

"I asked myself, 'How am I going to be able to carry the six hours with this interior woman, who speaks, but doesn't give away a lot?'" Kidman says. The answer came on the first day of production, from a director who knew how to use the full breadth of the actress's skills. "Susanne just said, 'I've got ideas.' And she literally had the camera in my face, or it felt like almost in my brain at times."

Key with Kidman, says Bier, is that "she can do every single thing with her eyes. With Nicole, it isn't just emotions — it's also thoughts. And this is where she's very, very different from everyone else. You don't just know what she's feeling — you know what she's thinking."

Grant felt the same authenticity when working with the Oscar winner. "There's always a thought in her head. She's just incapable of being fake," he says.

The two would regularly engage in improvisation on set, typically delivering a different performance — and different interpretation — with each take filmed. In one scene where Jonathan reveals an important revelation about his sister — an emotional release for him — Grace seems hardly moved, offering little response.

"Every single take was totally different," Bier says. "In one take, Nicole might be crying, and in another, she might be very cold. Or in one, Hugh might be crying, and another he's trying to kiss her," leaving a plethora of storytelling wealth to choose from in the editing room.