Most viewers of APEX—the new Charlize Theron thriller that began streaming on Netflix today—will have never heard the 2006 single “Nasty Boy” by the Icelandic electronic-pop/rock band Trabant before. But after watching Taron Egerton swing around on a rope like Tarzan, completely naked, while blasting a song about being a “a filthy little horny dog,” they won’t be likely to forget the tune any time soon.

“It was my favorite song,” APEX director and Icelandic filmmaker Baltasar Kormákur said with a laugh, in a Zoom interview with Decider. “I thought it was just a perfect song, and so many people responded to that. It’ll be a nice little find.”

Written by Jeremy Robbins, APEX stars Theron as Sasha, an experienced climber grieving the death of her husband (played by Eric Bana). She embarks on a solo hiking-and-kayaking trip in the Australian wilderness, where she meets a friendly local (Egerton) who offers her travel tips. But when she follows the route he suggests, she quickly realizes he isn’t friendly at all. He’s a deranged serial killer, and he wants to hunt the most dangerous game: her.

Theron, a seasoned action star known for doing her own stunts, leapt off cliffs into pools of water and dangled off the side of a mountain, hundreds of meters above the ground. Some of the movie was filmed on sets, but quite a lot of it was filmed in real, remote, and dangerous locations in Australia. “We had to swim to [some of those] locations, with the crew,” Kormákur said. “We dropped in, in helicopters.”

The director spoke to Decider about that on-the-ground location work, Threron’s jaw-dropping final climb, and, of course, Egerton’s “Nasty Boy” scene.

DECIDER: I know there are financial reasons to film in Australia, but a lot of movies would have Australia stand in for somewhere else, rather than make that the setting. Why did you decide to set the film there?

BALTASAR KORMÁKUR: Originally it was written for America—Yosemite, and the surroundings of that. I like that, but I’ve seen that a lot. It just didn’t feel as exciting. We had to shoot it in the South Hemisphere, because they were shooting it during the winter time—would be a way too cold water [to shoot in the winter in North America]. And then of course, the financial reasons. Australia came up. And then after delving into that, I said, “Let’s just do Australia for Australia—make that inform the story, rather than shoot it as Yosemite.”

That started a journey that I love. Let’s make our reality build the story, rather than the opposite. Then we found these Blue Mountains [a mountain range in New South Wales, Australia], which are crazy, those walls—I love the color of them and how exotic they are. Then I decided, let’s oppose that with with the coldest wall in the world, which is the Troll Wall in Norway. These [characters] are all over the world. Then casting Eric Bana, as her boyfriend, as the reason why she is in Australia. I love this part of filmmaking. Script is one thing. Then you have to create a reality out of it. If you have one [decision] inform the next one, you start on a journey that that is quite exciting.

Was there a specific location that you found that you were particularly proud of finding, and how did you incorporate that into the movie?

The cave—the setting, when they’re getting into the cave—we had to swim to those locations, with the crew. We dropped in, in helicopters. We basically had to scale us down to 40 people, the crew, and swim. When [Charlize Theron’s character, Sasha] goes up the two walls—that’s a location that the crew had never seen in, that this Australian crew had never heard of. They came to me at the end and said, “Wow. Thanks for taking us there. We’ve never seen this, and it’s in the backyard.” It’s a fantastic way to get to know a country, by the way, just to go to those places.

What, if anything, was shot in a studio or on a practical set in a water tank?

Some of the water work we had to do in a controlled environment. There is an Olympic kayak run in Penrith that we used quite a bit. But we also shot in crazy rivers, to create the environment and build it. Some of that was done in New Zealand, because we have better access to [rivers] there. We built walls to complement the walls that we were working on, so we could be more specific and get in the details. But that was already 20 meters in the air, 60 feet, when people were working on those. Even working on a wall outside, in a more controlled environment, you’re still faced with weather wind, and all those things. It wasn’t a secure environment. Then there’s the cave, and stuff like that, we did [film on a set]. But luckily, so far when I’m showing the movie, people cannot tell it apart. I do think it comes from doing the nature first, and putting the emphasis on that, and then implement it with spaces that you have to work in, rather than the opposite.

I love the music in this movie, especially the two songs that we hear Taron’s character listening to: “Go” by The Chemical Brothers, and this song “Nasty Boy”, which I had never heard before, but was perfect for that scene. Tell me about those song choices.

Originally in the script, [the line was] “Ten minutes, and I’ll come after you.” We came up with this idea that he was going to to put on a song, and when the song is over [he comes after her]. Then came all kinds of ideas of songs. I didn’t want a disco song or something like that. Taron brought this song by the Chemical Brothers, and I was like, “That’s perfect.” Not everyone was of that mind in the beginning. There was a lot of, “Oh no, no, no, it’s not the right song.” I said, we’re doing this, and then we can do another version if we feel like it’s that doesn’t work—try another song. Then Taron did [the scene] and the whole crew and everyone involved was like, “That’s it. It’s perfect.” They could see it like it worked.

The other [song, “Nasty Boy” by Trabant]—first of all, that scene was never in the script, that he was singing naked. I just saw that swing and said, “Wow. That’s great. Why doesn’t he play like a kid? Butt naked, swinging.” Taron was like, “I’ll do that.” This is an Icelandic song, actually, from an Icelandic band. Very famous artist, now—like a fine artist. [Performance artist Ragnar Kjartansson]. He was in a band when he was younger, and it was my favorite song in the ’90s. I thought it was just a perfect song, and so many people responded to that. It’ll be a nice little find. I just thought it was kind of perfect. I used to have fun in the bath with this.

It’s a great moment of levity in such an intense movie. So you came up with the idea for that scene where he swings naked? What was Taron’s reaction to that pitch?

I saw that swing in one of the locations. I saw some kids actually playing on it. It’s one of the less challenging locations to get to. I thought it would get great if while she is panicking, he’s just swinging and having fun. He knows she can’t get out—there’s no way out without a boat. He’s almost like a kid, drawing attention to himself with this. He’s like a kid who never grows up. A little bit of Peter Pan. [Taron] responded to that right away. A lot of the way we worked on it was like that, coming up with ideas that came from the nature of the places where we’re going to.

For me, the most intense sequence was Sasha’s final climb by herself. I read that final part of the climb was really filmed on the mountain. Can you tell me about filming that scene and directing Charlize through it?

I’m really happy to hear that. It’s not about chase, now it’s about surviving. I’ve seen quite a few films about climbing—movie stars hanging off and swinging and jumping and hanging on their knees, or whatever it is. It’s all light and fun. But for me, climbing isn’t like that. It’s really risky, especially if you’re free soloing. This is a life and death battle. You also don’t know what you’re climbing on, because these are rocks are not like a climbing wall. [A wall] is soft and you can trust it. The intensity is built from the details, for me. Her strain. How incredibly difficult this is, because we know that holding your body up like that, on your fingers, is not for everyone to do. That was what what we set the tone for.

Then in the end, she does that final ascent. It’s [Charlize], basically with hundreds of meters underneath her. I would almost have to crawl to her to give a direction, because I couldn’t stand there. It was such a dead drop. Her doing that was this a surprise to me, that she would be willing to do that. We could have done it in a different location. But she really wanted to do that. OK, let’s just do it. We found the location, so why not?

I know the movie wraps up pretty well at the end, but would you ever do a sequel? Have you and Charlize talked about that at all?

We haven’t really talked about it, to be honest. But you never know what lies ahead of you. I would certainly work with her again, and Taron as well. That would be something.