When it comes to any new X-Men movie, there’s always a question of which mutants we’ll see onscreen. The various characters from the comics have some visually stunning powers, offering the filmmakers of the franchise the chance to include dynamic, arresting imagery for each film, and given the bevy of choices at their disposal, one imagines choosing exactly which mutants to include is a bit tricky.

So when I was on the set of director Bryan Singer’s 1983-set sequel X-Men: Apocalypse last summer along with a small group of reporters, we were curious to know how the filmmakers go about deciding which mutants will be in each film. Apocalypse, for example, features returning mutants Charles (James McAvoy), Erik (Michael Fassbender), Raven (Jennifer Lawrence), Hank (Nicholas Hoult), Quicksilver (Evan Peters), and Havoc (Lucas Till), but introduces new mutants Apocalypse (Oscar Isaac), Psylocke (Olivia Munn), and Jubilee (Lana Candor) as well as new younger versions of previously seen mutants Angel (Ben Hardy), Storm (Alexandra Shipp), Cyclops (Tye Sheridan), Jean Grey (Sophie Turner), and Nightcrawler (Kodi Smit-McPhee).

So how does it work? Where does the process begin? Producer/screenwriter Simon Kinberg told us that on Apocalypse, it started with the villain and built out from there:

“With the main new characters, Jean, Scott, Storm and Apocalypse, those were very conscious decisions that we spent a lot of time considering, debating, and then making sure we had enough story to tell for those iconic characters, because when you bring in those kinds of characters you can’t just bring them in as a cameo; you really wanna delve into the characters. So the first decision that we made while shooting Days of Future Past is that we wanted this to be the Apocalypse story—obviously, because we had that tag at the end of Days of Future—and then we made the decision that we wanted a new sub-generation or semi-generation of mutants that were the young versions of the established characters of the original films. So that was the first wave of decisions, and then the characters you’re talking about like Psylocke, Jubilee, Angel, Nightcrawler, we sort of build it from the inside outward. We began with who’s the villain we want, what’s the story we want to tell for our four main characters?”

Above all, though, emotion and character are the foundational king:

“For me, Scott and Jean’s arcs are very connected to Charles and to Raven. Charles is the sort of father figure for them at the beginning of the movie, and Raven is ultimately a leader for them going forward. It has to all feel like it’s connecting emotionally, thematically. And then we build the story from there: What are the plot points of the story? When does the villain emerge? What does the villain want? How does the villain get in power? How does the villain get super-empowered and then taken down? That’s the basic plotline of every story in history, certainly superhero story. And then within that structure we start to think about building the teams, essentially, on both the hero’s side and the villain’s side.”

After the core characters are chosen, however, Kinberg says the process of choosing who to include becomes more visual-oriented:

“Then it becomes more subjective, random. Who are characters we’d like to see? What are powers we haven’t seen before? What are characters it’d be fun to see the young versions of, or the new visual effects version of? And then that’s how we start to fill in the ranks.”

The producer/writer used the addition of Olivia Munn’s Psylocke as an example, revealing that the character was a late addition to the Apocalypse ensemble:

“Psylocke was quite a late addition to the script and the movie. Bryan Singer and I were up here in Montreal and we felt like we needed a different Horseman, and we just started going through the cycling of the different Apocalypse Horsemen over history. We felt like we wanted it to be a female character and we pretty quickly settled on Psylocke. And super randomly I think a week or two earlier I was in Los Angeles and we were casting Deadpool. I had met with Olivia Munn for a character in Deadpool that ultimately wasn’t the right character for her, but we were like ‘We’ve gotta keep in touch, she has to do something in the X-Men world.’ And Bryan and I were sitting in Montreal a few weeks later and saying we should do Psylocke and I was like, ‘Dude, I just met with Olivia Munn two weeks ago. She’d be great.’ Then we looked at pictures of her online and I emailed her and I said, ‘I think this is a great character for you’ and she immediately emailed me back and sent me all this fan art online that fans had done of her as Psylocke. So that’s how that one came to be.”

But what of the other Horsemen? In the context of X-Men: Apocalypse, Oscar Isaac’s titular villain assembles a team of four villains to become his Four Horsemen, all in service of protecting and aiding him in his aim to bring order to the world. So how did Kinberg and Singer set about choosing who would join Apocalypse’s squad?

“Fassbender was the first person that we decided would be an interesting Horseman, and then truly we just kind of sat there the way that anybody would, rather you were in a role playing game or a video game or doing fan fiction, and it was like who are the coolest characters you wanna see? And who are interesting combinations with one another, and who are interesting foils for the heroes of the movie?”

Kinberg added that in choosing these characters, he and Singer focused more on thematic resonance as opposed to effects potential:

“A lot of the way that Bryan and I work together is it’s about theme and character more than story and spectacle. The story and spectacle comes and obviously there’s amazing teams here, and Bryan himself has such a cinematic mind, but we start it from the inside. So a lot of those decisions are made based on who are the people that deepened the core ideas of the movie? Who are the people that challenge, emotionally, the main characters of the movie, or are mirror images of the main characters? Things like that we take into account when making those decisions. Some from memory and some from Wikipedia (laughs), because it’s a long list and we really weighed all the options. When we work we also spend a lot of time going back and watching the different versions of the cartoons, the Apocalypse cartoons.”

This process of finding the characters and story of the film is a long one for the X-Men franchise, with Kinberg revealing that the conceptual phase of X-Men: Apocalypse was lengthy:

“We spent as much time on this in the conceptual phase as I did in the writing phase. And that was true of Days of Future Past. So I spent on Days of Future probably six to nine months in the conceptual phase and four to six months in the writing phase, and it was about the same on this because it was stretched over a year and a half. Getting to the shooting script in that year and a half I would say nine months to a year of it was in the conceptual phase.”

Clearly these X-Men movies aren’t simply assembled on a whim or thrown together with merchandizing in mind above all else. And with such rich history and a deep bench of potential characters, one could imagine other filmmakers becoming paralyzed by choice, having trouble zeroing in on who not to include as opposed to who to use in the film. So when you see X-Men: Apocalypse, know that every single mutant appearance was a heavily debated and considered choice, with plenty of potential characters left on the table for subsequent installments.

For more of our X-Men: Apocalypse set visit coverage, peruse the links below: