Peacock’s twisty and turny mystery thriller, created by Megan Gallagher and starring Sarah Snook, opens with a parent’s nightmare. When Marissa (Snook) goes to pick up her son, Milo (Duke McCloud), from a fellow parent’s house, she discovers that his playdate was a setup for an abduction.

Her realization doesn’t arrive as an avalanche. Instead, it’s a snowball, with the frightened mother quickly working through excuses and rationales, trying to find some — any — explanation for her son’s disappearance other than a kidnapping. Sam Williams, who previously edited “Big Little Lies” and won a BAFTA award for “Slow Horses,” worked closely with the first episode’s director, Minkie Spiro, to find the perfect balance of takes to capture Marissa’s slow, horrific awareness.

“Our first cut was very intense,” Williams said. “It went to 10 and stayed there. It really didn’t offer any variation of feeling and story to it, so that’s when we went back and refined it. It was more leaning into some of the takes where she was like, ‘No, maybe he is OK,’ so we could have that panic, the natural parent’s panic where they try to calm down. Like, ‘No, I’m just being unreasonable here.’ And then [the fear] would resurface.”

Working with a powerhouse like Snook, Williams had no shortage of options to choose from. “Editing, for me, is a lot about what the actors bring to the performance,” he said. “Sarah Snook is going to do this, and really, my world is trying to not get in the way.”

The opening sequence not only serves to hook viewers but also sets the tone for the seven episodes that follow. So Williams homed in on the most direct, efficient, emotional story possible. He gradually amped up the tension by using shots that grow tighter and tighter on Snook’s face, leaving her character and viewers feeling boxed in.

“Anyone who’s edited an Episode 1 knows that the pressure’s on to make sure everyone’s established straightaway,” he said. “That doesn’t mean it gets easier in Episodes 2, 3 or 4, but 2, 3 and 4 are going to suffer if you haven’t set the characters up well.

That’s something Williams would know, seeing as he also edited the third episode. (Paul Knight, Dan Roberts and Melanie Viner-Cuneo handled the other six.) By the time that third hour rolls around, “All Her Fault” has started playing with the timeline.

Williams had to find the right rhythm between the present-day crisis and flashbacks to the Chicago Marathon two weeks prior, as recalled first by Milo’s babysitter, Ana (Kartiah Vergara), and then by Marissa as they talk to detectives. A detail from Marissa’s recollection introduces a key plot point that the narrative returns to in future episodes, slowly feeding more information to the audience

“That’s always interesting, when you’ve got elements they don’t know about,” Williams said. “Episode 1, you’re trying to set up people in one light, and then you’re just peeling back that onion the whole time.”

This story first ran in the Limited Series/TV Movie issue of TheWrap’s awards magazine. Read more from the issue here.