They’re the men behind the monsters.

And while you might not know their names, you certainly know their work. Between them, animatronics designer Gustav Hoegen and prosthetic makeup artist Conor O’Sullivan have informed the creature creation process on practically every genre franchise you could name.

Decades into a career that’s seen him design live Engineer heads for Prometheus, Vogons for The Hitchhiker’s Guide to the Galaxy, and Admiral Raddus for Rogue One: A Star Wars Story, Hoegen has taken up permanent residence in the galaxy far, far away. Since first partnering with Lucasfilm on The Force Awakens, he’s worked on every new Star Wars film and the upcoming series Andor.

O’Sullivan, meanwhile, helped to create the White Walkers for Game of Thrones, Beast in X-Men: First Class, and the Xenomorphs in Alien Covenant. He’s earned two Oscar nominations, for Saving Private Ryan and The Dark Knight; for the latter, he created the Joker’s worn, cracked makeup and fashioned his facial scars from silicone molds.

Though Hoegen and O’Sullivan have teamed up in the past, most extensively on the Alien movies, their latest collaboration was something a little different. For new Finnish horror Hatching (now in theaters; on VOD May 17), both worked closely with director Hanna Bergholm to create the viscid, gangly creature that 12-year-old Tinja (Siiri Solalinna) inadvertently hatches from a large egg she discovers in the woods by her suburban family home.

Secretly rearing the creature, which Tinja names “Alli,” gets more complicated as it starts to evolve in odd, frightening ways. To capture the creature’s creepy appearance and its growth from a monstrous bird-like being into a humanoid doppelganger, Hoegen’s Biomimic Studios and O’Sullivan’s Creatures Inc. joined forces — with blood-curdling results.

“Even with details like the eye color, we were making sure we were on the same path, so the creature would also appear to be evolving throughout the film,” O’Sullivan tells Inverse. “I was on the more human side of it, whereas Gustav was on the creature side.”

In an interview with Inverse, the pair discuss their careers in monster-making, why Alien continues to inspire them, and the “most stressful” productions they’ve ever worked on. (Hint: O’Sullivan isn’t exactly itching for Morbius 2.)

This interview was edited for clarity and brevity.

Inverse: I know that Hatching’s director, Hanna Bergholm, was closely involved with crafting the look of Alli, the film’s creature. At what stage did you become involved?

Gustav Hoegen: Hanna spent time with concept designers nailing the look, so what was presented to me was quite exceptional. On some films, they have a little scribble or a description of what they want. In Hanna’s case, she had beautifully rendered drawings. She even had a 3D spinning model of Alli, with all its contorted shapes and ribcage. All I had to do was blow it up on several pieces of paper for size reference; its size had to be the same as Tinja standing up — one meter 20 cm, roughly. That’s what we stuck to; we copied what was presented to us. But it doesn’t stop there because all the nuances come in.

Can we have a more protruding ribcage? Can we have a shorter beak? A bit more texture, here and there? They’re all small details. If I would pull up all the emails discussing the look, I’d probably end up with 300 emails: the look of the tongue, the color of the eyes, the size of the pupils. All these details still take back-and-forth, to lock them down, but the overall look was very well-prepared.

“Keep it as close to reality as you can.”

How did you approach constructing the creature? Alli has this twisted, misshapen appearance and bird-like movement that’s very memorable.

Hoegen: We’re on a budget, but for a complete creature, the go-to method is a rod puppet. That’s an ancient form of puppeteering. Even in the far East, they have these shadow puppets. This is a more advanced version of that. It can really free you up in terms of the anatomy of the creature; you can give it a very big head because you don’t have to worry about internal mechanisms holding the weight. It will be externally operated by a person, holding the creature by sticks.

Conor O’Sullivan: Gustav was involved with the puppetry, and I was involved with the prosthetics. Though we were looking at each other’s work and trying to work out how to make things best fit in with each other, we were there at separate times. Gustav would send me pictures of what he had made; I would show him what I was making.

Hoegen: Conor offered me the usage of his workshop for pouring the skins and baking them. It was a very good collaboration, with no secrets.

O’Sullivan: I wouldn’t have any secrets from you, Gustav. [both laugh] When I got around to starting, it was later than Gustav, so I was able to see what he had made as far as the creature, sculpturally, and that influenced what we were making. It was more about making the two marry and look like a continuous evolution.

Hanna Bergholm has spoken about her desire to create an “iconic” creature. What’s the secret to creating those distinguishing features that linger in the imagination?

O’Sullivan: Keep it as close to reality as you can. I remember my father once told me, if you’re going to lie, you’ve got to keep it as close to the truth as possible. He lied frequently, but I think that’s actually quite a good motto.

Hoegen: I have to give credit to Hanna and her design, which was prepared so well. My input was moreso technical in how to bring it to life. Cosmetically, we worked closely, and she clearly had a finished idea of the creature in her mind. You give yourself freedom here and there, but every detail was covered, down to the look of the ribcage, the skin, and the side of the tongue. She was meticulous.

Is it unusual to have a director prepare designs in such detail?

Hoegen: It can depend. On Star Wars movies, for example, they would just present you with a script and leave it to us what creatures would inhabit that story. You’d have a design team just pumping out design after design to plant a seed in the director’s mind. And he would pick the ones he liked and leave it to us to build it. But then, in a case like Hanna, everything is completely prepared for you.

O’Sullivan: I did Morbius, and I must have produced 500 or 600 designs. And at the end of all of that, they threw something on the table — literally the day before we started shooting — that was completely different from the path that we’d been on. So, it ended up digital because Sony wanted to do this completely different design. And now they’ve brought the film out, and it’s gone back to all my designs, which have all been done digitally! [laughs] We do like it when a director comes to the table with a full idea of exactly what they want. It’s nice to contribute designs, to be part of the process of developing them. It’s also nice to be told what to do.

“That didn’t go down very well with Christopher Nolan.”

Conor, the facial deformities that develop around Alli’s mouth later in Hatching — the torn-open jaw that reveals human canines — reminded me of your work on the Joker in The Dark Knight. How do you approach creating makeup effects that feel realistic and visceral in that way?

O’Sullivan: Julian Murray was the sculptor on Hatching, although I’d done the Heath Ledger sculpture on The Dark Knight, for those scars. The initial design input from Hanna gave the description of a bird that was changing into a human. We were at the stage with a beak where you could start to see the human elements coming through. It was built around a human. The creature rips the beak off, and you see the teeth and the mess inside, but you also see human teeth emerging. As far as the prosthetics are concerned, technically, they’re fairly standard. Getting it to look real is something all of us are trying to achieve.

Julian did a beautiful job of sculpting the pieces. He is a classic sculptor from the Harry Potter movies. The first time I worked with Julian was on the Joker. He designed Two-Face directly with Christopher Nolan before I turned up. I remember the visual effects department tried to redesign it, and that didn’t go down very well with Christopher Nolan because he’d spent about two months working personally with Julian on the Two-Face design. We got a hold of Julian’s sculptures, molded them, and then produced finished copies in silicone for the visual effects department. They were lovely.

Gustav, Alli has a dramatic presence in Hatching. You’ve created such creatures before. Admiral Raddus in Rogue One, for example, was yours. How does your approach change when your animatronic has to deliver dialogue or carry scenes?

Hoegen: What is great about characters such as Admiral Raddus or Alli is that, when the sculpt is great, it does most of the work for you. With the Engineer’s head or David’s head in Prometheus, for example, they are realistic human heads and also very recognizable to you or me. Creatures, in a way, are a lot easier. What we tend to do is make the sculpt as expressive as you can. A lot of the character of it is achieved by doing that, and we enhance those characteristics.

“For Admiral Raddus, I might look at a bulldog.”

Obviously, I borrow from what I see around me in nature. For Admiral Raddus, I might look at a bulldog, with its jowls, and try to emphasize that. We based him on Winston Churchill, so we made his jowls quite heavy, made him wobble around, and everything is quite plump. He’s a commander of the fleet and an authority. That’s how I approach each character individually. I find something — either a person, a historical person or an animal — that shares those specific characteristics and try to translate it through my animatronic.

That’s quite important. You’re not just making a mouth go up and down; you also have to give it character. It’s very important to focus on the actual creature and not get lost in the technicalities of it because then you take your eye off the ball. It’s all about what you see on the big screen eventually. If that doesn’t work, you failed.

Conor, you were a prosthetic supervisor on Prometheus and a creature design supervisor on Alien: Covenant. Gustav, you were a senior animatronics designer on Prometheus. What does the Alien franchise mean to you both?

O’Sullivan: On Prometheus, I was totally responsible for the Engineers, plus an aging [effect] on Guy Pearce. They initially didn’t believe that Engineers could be done with prosthetics, but Ridley had come across some work of mine and was interested in it. At the time, I was pretty excited. I loved the design. I thought it was absolutely fantastic. And Ridley knew what he wanted.

Then, I was thrown to the wolves on Alien: Covenant. Trying to follow the initial franchise of Alien was quite a frightening prospect. Initially, I was told that all the designs were done. Two months before shooting, on the way to China to do another job, I got a message asking if I wanted to be the film’s creature designer. And I was like, “Really? I thought you had all the designs done.” Then, I discovered that none of the designs had been done.

“Alien was one of the only reasons I got into this business.”

Everybody was terrified. I had people designing stuff all over the world, trying to come up with ideas, because it was quite a significant move away from the original. Everybody’s got that H.R. Giger look entrenched in their minds and believes in it, but Ridley wanted something much more biological, rather than biomechanical.

That was quite frightening. But Alien was one of the only reasons I got into this business. It was Alien that really woke me up to the idea of animatronic work and prosthetics. I’d always done sculptural molding and ceramics, but I wasn’t that focused on the world of creatures in films until I saw Alien. To me, Alien was like opening up Tutankhamun’s tomb. Watching that film would have been the same feeling, as finding the only intact tomb of the Egyptians that you could have ever seen. I don’t think you could put it into words.

Hoegen: The Alien franchise was very pivotal for me as well. It’s one of the most influential films I’ve seen and part of why I’m doing this. I was nowhere near as high up as Conor was on the films, but I came onto Prometheus to do animatronics and saw that as the pinnacle of my career. This made me put a lot of pressure on myself. I have mixed feelings looking back on that film because obviously to be on an Alien movie directed by Ridley Scott was the best thing I’d done. Being allowed to do an Engineer’s head was incredible. And the David head, I always thought would be the coolest thing to do; it was like the Bishop head [from the original Alien] but made mechanical.

I was on cloud nine for a while, but then reality sets in and you actually have to produce something they can use. It became a very anxiety-riddled project, and I wasn’t even in the position Conor was in, which is way more stressful. I look back at it with joy and with a lot of pain — mainly pain. But it was all self-induced. It’s all about how much pressure you put on yourself and what goes on in your own head. Most people thought it was a fun, relaxing job.

O’Sullivan: Prometheus, for me, was one of the most enjoyable jobs I’ve done. [Hoegen laughs] My world was simpler than the world of [creature and special makeup effects supervisor] Neal Scanlan. I was in his position on Alien: Covenant. Neal was Gustav’s boss, and they were running the creatures department, working on the Deacon and the larger Trilobite, whereas I was much more focused on the Engineer.

It was already designed, it was beautiful as it was, and I just had to technically deliver it. I had a lot of good sculptors and molders working on that, so Prometheus wasn’t too stressful. Alien: Covenant was a completely different role, and I would say it was the most stressful thing I’ve ever experienced in my life — apart from Morbius.

Hatching is now playing in limited theaters and hits VOD on May 17.