'You' stars and creator share 13 things even superfans may not know about the Netflix series

Speleers, who played author and mayoral candidate Rhys Montrose, told Insider that he knew prior to taking the role that his character would primarily be a manifestation of Joe's (Penn Badgley) darkest thoughts.

In fact, it was "one of the selling points" for him. Still, a lot of the cast didn't know so he had to keep the twist "very, very quiet" and he wasn't sure exactly how the plot would play out, as he wasn't provided all of the scripts ahead of time.

He added, "I just had to be forward-thinking in terms of what was coming."

"So there still has to be some sort of just a little nod here and there," he said. "Just to tee it up for what happens later on."

Although many scenes featured just Rhys and Joe, a handful required special angles and acting in order to not give anything away. After all, Rhys was only supposed to be visible to Joe.

"We have to be very careful as a group of what not to look at," explained Speleers. "'Don't look at Rhys, don't look at Rhys.' There was a lot of that going on."

Sera Gamble, cocreator, executive producer, and writer for "You," told the cast to think about "Fight Club" during filming. In the 1999 movie, a character is revealed to be a figment of the protagonist's imagination.

On episode nine, directed by Badgley, Marienne fakes the loss of her daughter. Gabrielle said it was one of the most difficult scenes to film.

"I really struggled to wrap my head around it, and I was getting so frustrated that day that I felt like I couldn't get to the point that I wanted to get to," she continued.

She said they went through "so many takes" and her costar Badgley helped her get through it.

"I feel like I would have had a very hard time with that episode had I not been directed by Penn," she said, adding he was a "wonderful director."

"We road-tested a version where he was entirely imaginary," Game said. Instead, they opted to make Rhys a real figure whose presence was manifested by Joe.

Gamble told Insider that Rhys was always going to be a figment of Joe's mind "like a 'Fight Club' story" but Rhys' character evolved over time.

"We worked backwards from 'he doesn't exist' and then we realized, well, yeah, he does — he just has never met Joe," she added.

Some fans theorized that Love Quinn (Pedretti) could still be alive despite being murdered by Joe at the end of season three.

But Gamble told Insider there's no question about it — Love is dead. Her season-four cameo was, in part, to help confirm it.

"No matter what I say, people say, 'You're leaving a little loophole about whether she's dead.' Didn't you see her dead body and then the whole house blows up?" she told Insider. "And they're like, 'That's not good enough.'"

She said cocreator Greg Berlanti is to thank for pitching cameos from Pedretti and Elizabeth Lail, who played Joe's first love interest, Guinevere Beck, on season one.

The actor told Insider his favorites scenes were where "a lot of the scheming took place."

"I found some of those scenes in the apartment a lot of fun to play because it was just me and Penn, bouncing around this space," he said. "I'll be popping up in different elements, different parts. You know, next to the camera, behind the camera, in front, all over the place."

He said it was "fun" to "put Joe off and completely toy with his mind."

According to Gamble, creating a murder mystery is "not easy" and season four was "the most difficult season so far" to write.

"Writing a long whodunit is trickier than I thought it would be. I've only ever done them for a single episode before. I've never done it for five, and it turns out that's a challenging, beloved but challenging, structure," the showrunner told Insider.

You also have to add another layer by going back to "track all of it and make sure that your protagonist could really be the murderer," she said.

The cast and crew expected a chill and fog but ended up filming during a lot of heat.

"We were supposed to shoot outside on the hottest day on record in London," Gamble told Insider. "The thing we did not expect when we went to foggy London was that we would be there and the equipment would be melting because it was over 100 degrees."

Gamble said it can be tough to hide spoilers and avoid paparazzi when shooting scenes, especially ones that are outdoors.

She said some British papers snapped photos of Badgley and Speelers on the bridge while they were filming the scene when Joe debates on jumping.

"I'm not tracking this closely, but I hope nobody figured out exactly what was happening there. It looks like two guys having a very serious conversation, and yes, there were stunt doubles and some of it was shot on a stage," she said.

One of the show's recurring sets — Joe's glass box in which he keeps his captives — is "the most challenging standing set to shoot" with, according to Gamble.

Because it's essentially "a big reflective box" it's hard to not get equipment and crew to appear in the glass.

"It took a long time to build and to test and to figure out how to not have every scene just be a scene of you watching camera crew," she told Insider.

Lady Phoebe's (Tilly Keeper) estate, Hampsie House, was filmed on location at Knebworth House in England's Hertfordshire. The estate is commonly used as a filming location and has been featured in films like "The King's Speech" and shows like "The Crown."

The historic structure couldn't be subject to real flames, so Gamble told Insider the crew had to get creative to pull off the season's massive fire sequences.

"The fire outside was completely visual effects," she said. "There's a lot of interactive light and smoke that went into the scenes in the cellars ... but a lot of it was put in during post-production."

As for the dungeon scenes, Speleers told Insider that they were filmed at a studio, not at Knebworth house.

Gabrielle spends a lot of the season trapped in Joe's infamous glass box. To prepare, she told Insider, she'd spend anywhere from six to 12 hours a day in the glass box, often alone.

"I really loved our crew for how supportive they were for me in this box. They wouldn't come in, so I really got to feel the sense of full isolation for about a week and a half, maybe two," she said.

Gabrielle said she actually ended up getting comfortable in the glass box and would only leave it when she had to use the restroom or take a lunch break.

"It became like my own little personal studio apartment," she joked. "I had a bed in there. I would keep my headphones and my phone underneath the mattress. We put a water in what was Marienne's toilet in the box, so I had everything I needed."

Season four ends with Marienne's escape and reunification with her daughter. However, this ending isn't enough for Gabrielle, who told Insider she hopes Marienne gets revenge on Joe.

"I think that her moral compass still feels off in a way and she is still rubbed wrong by the fact that Joe is still out there and he's not experiencing any kind of consequence for the things that he's done," she said. "So, I would love to see Marienne be the one to go after him."

She also thinks "death is too easy of a punishment for Joe."

"He deserves to suffer a bit and be stuck in a cage by himself for however long, and I would love to see Marienne be the one to exact that," she said.