In Tobias Lindholm’s Netflix drama The Good Nurse, Eddie Redmayne plays real-life serial killer Charlie Cullen, who at first seems like a sweet, docile hospital nurse until it’s revealed over the course of the film that he’s been stealthily killing perhaps hundreds of patients — for no apparent reason. Fellow Oscar winner Jessica Chastain stars as Cullen’s colleague Amy Loughren, who discovers Cullen’s crimes and fights a corrupt medical system to expose him.

Loughren’s fight served as the subject of Charles Graeber’s nonfiction book of the same name, which became the basis for the script by Krysty Wilson-Cairns (a best screenplay Oscar nominee for 1917). Loughren met with both Redmayne and Chastain prior to filming and even visited the set. After six years of production, with stops and starts as funding came and went, her story is finally out in the world. It debuted at No. 1 on Netflix’s English-language films list, reaching the top 10 most watched in 93 countries and garnering particular praise for Redmayne’s haunting performance.

THR spoke with the actor about preparing to take on such a daunting role, what it was like getting into the head of a killer, working alongside such a formidably talented co-star, and the experience of speaking with the real Loughren ahead of filming.

I understand the filmmaking was a six-year process. What was your first impression of the script, and what caused it to take so long?

I knew nothing about the story, which was the best way to go into this. As Krysty Wilson-Cairns’ script unfolded, what I found extraordinary about it was it didn’t seem to fit comfortably into any specific genre. I knew that Tobias [Lindholm] was directing it, and I’ve watched his work, particularly A Hijacking and A War, and was just so astonished by his vision of the world. And getting to spar alongside Jessica, who I think is one of the greats — that combination felt extraordinary. I was very quickly in, but then the studio let it go, and we were trying to reconfigure it somewhere else. Even though our schedules kept not matching, we remained passionate — I’m so thrilled that we did stay with it.

I want to hear more about the preparation — you guys went to nursing school?

Tobias, as part of his process, likes to insist on a month’s rehearsal. Two weeks of that was, in the mornings, Jessica and I going to nursing school, which was this ICU pediatric nurse, Joe, teaching us about the history of nursing. Then it was a lot of learning to do CPR, learning the language, more than anything, and how not to overemphasize, how to understate, in order to not worry people in moments of crisis. It’s a very assertive walk, retaining a certain calm. Technically, for me, it was about trying to get Charlie’s voice right. There were a lot of recordings that I had. There’s this wonderful woman called Alexandra Reynolds, this dancer who I first worked with on The Theory of Everything, and she and I [looked] through all the footage we could find of Charlie. He’s always self-soothing. On the surface, he seems still, but if you look closely, there’s actually a lot of gentle movement going on. There’s this blankness to Charlie.

The movie is only a third of the book, which gets more into Charlie’s backstory. What elements of the book did you find most useful?

He came from a large family, and his father died in his first year. His older siblings had partners, one of whom abused him when he was 7. And he tried to kill that person with, I think, a kids’ chemistry set. When that failed, he tried to kill himself with lighter fluid. When he was 15, his mother died in a car crash. He was the first person to the hospital, and they’d lost her body. There was no respect there. He was incredibly bright, and he passed all the psychiatric tests to join the Navy. He was bullied. Then he was found with his fingers over the missiles on the submarine, and that is when he was removed from working in the Navy. After that, he went back to the hospital that his mum had died at to train to be a nurse. He then was in and out of institutions while working as a nurse. He stalked another, female nurse.

You got to meet the real Amy. What was it like getting to talk to her?

She talked at length about her love for Charlie — how kind he was, how empathetic he was, what a brilliant nurse he was, how bright he was, how self-deprecating. And how he saved her life. And then she said, “I met this different human being twice,” and she described how something came over his eyes, and one eye drifted, and there was an arrogance and a truculence. She described it as dissociative. But it was her reminding him of his humanity in those moments, when he was this other human being, that brought him back.

Eddie Redmayne as Charlie Cullen in Netflix’s true-crime drama The Good Nurse.

Eddie Redmayne as Charlie Cullen in Netflix’s true-crime drama The Good Nurse.

Courtesy of JoJo Whilden/Netflix

Charlie killed people and did evil things, but I think the movie is still not afraid to acknowledge that he is a person and there was good in him. I’m curious if you came to a conclusion about why he did it.

He may have done monstrous things, but he was and is a human being. That, I suppose, ties into the why of it. What I found riveting about the script is it didn’t answer that question. But as an audience, we need that question answered because we need to know that he is “other” and he did something that we would never do — he did it for a reason. If we have that reason, then we can feel safe. But actually, that’s about us rather than about him. What I love is that Tobias wouldn’t give that answer because there is no answer. Charlie has stated to begin with that it was mercy killings. And he did work in burn units, and they are horrific places, and the euthanasia part of it one could begin to believe — except for the fact that he ended up then killing hundreds, maybe, of people whose health was improving, and he also wasn’t specific with who he was murdering.

It sounds like you and Jessica had a great time, even though it was really heavy subject matter.

I’ve got young kids, and we all went to New York and Connecticut to make the movie, and she has a young family, and they were incredibly generous. The weekends we would hang out, our kids would play together. The work itself was intense, no question about it. Jessica would always make sure that there was some tequila on hand on a Friday night; Tobias, the three of us would go and toast the weekend. She was also busy producing or doing script notes on George & Tammy [Showtime’s upcoming limited series starring Chastain as Tammy Wynette] at the same time. You’d hear her practicing singing next door. Her capacity to multitask whilst being entirely present in the character was a stunning thing to witness.

What’s lingering with you from this experience now that the film is out?

The film reminded me of the power of the individual. Sometimes you go, “Well, you know, we’re just in the system. There’s nothing we can do.” But [Loughren’s] capacity to confront that, and the humility with which she did it, was quite something.

Interview edited for length and clarity.

This story first appeared in a December stand-alone issue of The Hollywood Reporter magazine. To receive the magazine, click here to subscribe.