Ari Orphan Black Ari Millen will appear in "Orphan Black" Season 3 as Project Castor clones, but he almost didn't make it through Season 2. Find out why the creators didn't kill Mark below. Photo: Steve Wilkie/BBC America

"Orphan Black” introduced Project Castor, the military operation with male clones, at the end of Season 2. Ari Millen’s character, Mark, was revealed to be a clone, and he’ll be back for Season 3. However, that wasn’t always the case. Millen wasn’t supposed to be in "Orphan Black" Season 3 at all.

Executive Producer and Co-creator Graeme Manson said that while Mark wasn’t planned to be a clone, Project Castor had been in the "Orphan Black" plan for some time. “We had known that we wanted to do Project Castor and expand our conspiracy, that it was an essential part of the conspiracy,” Manson told TV Line. “We’ve known about it—I don’t know about all along, but we’ve known for a long time. We just didn’t know who it was going to be.”

Manson explained that Felix was a choice that was “too obvious,” and Mark wasn’t supposed to be a clone. He was supposed to die.

“I think we thought that we would probably just cast someone new, that we would find a new person,” Executive Producer and Co-creator John Fawcett explained to TV Line . “As the season kind of developed and we were trying to figure that out, we realized that [revealing a new actor] just wasn’t going to be that satisfying. And we started to look inward at the assets we already had, and Ari Millen was cast as Mark at the beginning of Season 2 and was sort of slated to die partway through the season. And we just kept being kind of mesmerized by the work he was doing and kind of went ‘Shit. We can’t kill this guy.’ Then we kind of went, ‘Wait a second.’”

Manson added that the reveal of Mark as a Project Castor clone was a great way to close the season. “We realized just from a pure storytelling point of view that the face that Sarah sees at the end is the same face that sits across from her at the diner in the opening sequence,” Manson said.

Don’t think that the Project Castor clones are separate from the Project Leda clones, though. “We’re telling the mystery puzzle of Sarah Manning and her clone sisters and Project Castor is another part of the mystery,” Fawcett explained.

Project Castor boys will be back, but they won’t be the only male clones sticking around. Manson also mentioned that he was interested in bringing back Tony, the transgender project Leda clone. He and Tatiana Maslany both want to explore the character, and Fawcett shared their feelings. “We’re also interested in the relationship between Tony and Felix. That’s something that I think still has a lot of legs,” Fawcett said.

Watch the full interview below:

“Orphan Black” returns to BBC America in 2015. Are you excited to explore the Project Castor clones? Sound off in the comments below.