The actor appeared in the season 6 spacefaring tragedy, “Beyond the Sea.”

Black Mirror doesn’t just traumatize its audience, it also leaves a dark imprint on its stars.

Josh Hartnett is still recovering from the nightmare fuel explored in his one-episode stint on Charlie Brooker’s sci-fi anthology. The Penny Dreadful alum starred in the season 6 episode “Beyond the Sea,” which follows two astronauts who are in the midst of a long-term deep space mission when tragedy strikes and Hartnett’s David must watch the murder of his wife and children.

For a married father of four, inhabiting that dark headspace took a toll on Hartnett.

“We did that for two nights, and it was really horrible,” Hartnett said during a Variety profile interview.  “It was horrible to imagine because that’s the only way I can do it. I’m not much of an actor. I have to really put myself in a situation.”

He added, “That’s the job. Sometimes you have to go to places you don’t want to go to.”

Hartnett explained that while he typically enjoys playing layered characters who experience “things that are way outside of what I understand,” David’s dark headspace was particularly difficult to shake.

“I’m not great at getting out of it, especially if it’s hard to get into. It takes about the same amount of kind of effort to come out because I can’t just pick it up and put it down,” he said. “At the same time, I’ve got amazing friends, I’ve got an amazing family, and I have kids and they don’t care what I’ve been doing at work. They need things, so I can’t really bring it out. It’s not an option.”

Though the Charles Manson-inspired murder of David’s family was particularly memorable, “Beyond the Sea” had plenty more dark twists in store for its audience. The episode — which also stars Aaron Paul and Kate Mara — features technology that allows astronauts to transfer their consciousness to replicas of their bodies located on Earth. In true Black Mirror fashion, this is eventually used to horrifying effect. But before David spiraled down a dark path, Hartnett saw him as a hopeful character.

“I think that David is 100 percent an optimist who is hopeful about the future of technology and its relationship to people,” Hartnett told Entertainment Weekly in 2023. “And for that sin, Charlie Brooker had to destroy him.”

Hartnett added, “David represents a time and place where there was great hope in our future outside of Earth. Colonizing the moon, or even Mars, that sort of thing didn’t seem like a distant possibility. In our reality, this was the time of the Gemini space program and people going to the moon. It was all happening! But this is Black Mirror, so everything goes very, very pear-shaped after that.”