In Apex, Charlize Theron and Taron Egerton’s characters don’t exactly get along. Theron plays Sasha, a grieving mountain climber heading out on a vision quest of sorts in the Australian Outback. Egerton is Ben, the local depraved serial killer who hunts and traps her for sport. Crossbows, underwater wrestling, and a chase through treacherous woods ensue.

But when the cameras aren’t rolling, the stars of director Baltasar Kormákur’s new thriller have nothing but the utmost respect for each other.

“Charlize is probably one of, if not the, hardest‑working actors I’ve met,” Egerton told Netflix.

“Taron Egerton is one of the top-five best actors I’ve ever worked with in my entire life,” Theron also told Netflix.

That mutual admiration doesn’t mean filming Apex was a breeze. “When you don’t have obstacles, filmmaking becomes too contrived, too easy, and nothing feels real,”  Kormákur (Beast, Adrift) tells Netflix. “The obstacle is what you’re looking for. The rub between your idea and the obstacle is often where art is created.”

That philosophy carries through to the film itself, which is as concerned with what this situation reveals in Theron and Egerton’s characters as it is with being a survivalist thrill ride. “It’s really a story about survival, not just physically but emotionally,” Theron said. “[It’s] about finding out what you’re made of.”

For Theron, a former ballet dancer and action-movie veteran known for Mad Max: Fury Road, Atomic Blonde, and The Old Guard films, Apex demanded a whole new level of physical endurance — although she’s careful to add it came with help from the film’s stunt team.

“Anything you see of me going down a waterfall or some of the really dangerous rapids in nature, where we didn’t know if there was a rock underneath, I had two incredible women, River Mutton and Luuka Jones — world‑class, Olympic‑level kayakers — doing that for me,” she noted. “The climbing, though, I think I did all of it — and I loved it.”

Theron, who also produced Apex, trained with accomplished rock climber Beth Rodden to play Sasha. “Hearing her story and her expertise, I thought, this is the most badass, best climber you could be with,” Theron said. “She’s the OG.” The pair built up to the barefoot, jeans-clad climbing seen in the film with plenty of gym prep. “Beth really embraced how much I loved it and even offered to take me bouldering with her,” Theron added. “Our kids are around the same age, so the four of us would go climbing.”

By the end of her training, Theron had discovered a new passion for what she calls “a real art” of its own, and even found parallels between climbing and acting.

“The problem‑solving necessary for both is very similar. I went into it thinking Beth was going to teach me a dance — ‘then there’s this step, then this step’ — and it really isn’t that,” Theron said. “She was like, ‘Well, you can do this, or you can do that, or you can just feel if it’s something in between.’ That’s really what acting is about.”

“There’s a mental problem‑solving component that my OCD brain loves,” she continued, “and I love that it requires this all‑over body strength.”

Egerton’s role, meanwhile, required a very different kind of strength: the boldness to play a true psychopath, with no frills. It was also an opportunity for the actor to stretch beyond his comfort zone after turns in the Kingsman films and Netflix hit Carry-On solidified him as a lantern-jawed force for good.

“The actors I most admire often change lanes a lot,” Egerton said. “Carry-On was a wonderful experience and a great success, but I always felt that character was quite straight — quite blue-collar, with a procedural element. I had a strong desire to rough up whatever image that character created, especially since it’s the thing I’ve done recently that reached the widest audience.”

Enter Ben, a psychotic killer who delights in toying with his victims like a cat with a mouse. “The extremity of the role really appealed to me,” Egerton said. “Ben is just about the most fucked‑up character I’ve ever played. There was something delicious and irresistible about it.”

And he relishes the prospect of his character terrifying audiences.

“I kind of think that’s what I was being paid for, so that’s a good thing. Ben is supposed to be arresting, supposed to be scary,” Egerton said. “But I think with good villains, there’s always a part of you that empathizes, or at least recognizes their humanity or how broken they are. I would never excuse the monster that he’s been, but I think there are enough clues in the storytelling to know he’s a pretty broken individual, someone beyond rescuing.”

Egerton worked with Kormákur and screenwriter Jeremy Robbins (The Purge) to shape the character of Ben to his own liking. “The role was in flux, and everyone was very open to finding whatever version of him grow organically out of our shared conversations,” Egerton said. “When I was originally sent the script, he was more of an archetypal ‘wild man of the woods’ with a full beard and top knot — that rugged, one‑with‑nature guy. I read it and thought, there’s a Joel Edgerton who could probably do this, but this is outside my remit.”

Instead, the Ben that emerged in Apex was an unsettling fusion of man, beast, and Ben’s own past, the young man who never escaped his traumatic childhood. “I suggested we massage it into something that felt more like a kid playing pretend,” Egerton said. “[He’s] a kid trying to demonstrate that he’s a grown‑up and knows his way around these parts.”

Egerton had his own level of physical and mental preparation to complete for the role before he headed to the Outback with Theron and the crew. “This one was less about, ‘How do I want to look?’ and more about, ‘Am I going to be able to do what’s required?’ ” he said. “As the actor, you have to make [your] role the hero of the story. In my version, Ben is the hero, and everything has to make sense to him. Part of my job was to craft not exactly a logic, but a kind of spiritual code for him that I could refer back to when I didn’t know what choice to make.”

Once Egerton and Theron finally met, they were off to the races, each delighted to find an equally committed screen partner.

“I knew he was good — that’s why I wanted to work with him — but I had no idea he was this good,” Theron said. “He is insanely good, not just as an actor but as an artist with real intuition and a very trustworthy gut reaction to character and story. He can see things from 30,000 feet in the air and also be right there in the tiny detail.”

Speaking of being a number of feet in the air… “Our first week in Australia, I was working half the time and having quite a nice time on Bondi Beach,” Egerton said. “But Charlize was out in the wild, doing this sequence before she meets me by the river. In the script it said she leaps from a great height into a river, and I just hadn’t processed that she would actually do it for real. Balt showed me the footage of her doing it — cool as a cucumber. She did it about five times. I was so impressed. It really set the tone.”

Sasha’s survival in Apex, like Theron’s performance, like her leap into that river,  came together one step at a time. “In both climbing and acting, you’re constantly making choices, adjusting, responding to what’s in front of you and what your body is telling you,” Theron said. “You’re using technique, but you’re also relying on instinct.”

Apex is now streaming on Netflix.