‘The fire is a character’: why the terrifying infernos of Those Who Wish Me Dead look so realistic

For Angelina Jolie's latest film, in which she plays a wildland firefighter – a “smokejumper” – the screen needed to fill with fire. Those Who Wish Me Dead, based on Michael Koryta's 2014 book of the same name, sees Jolie's character Hannah trying to keep a young boy, Connor, safe from the hitmen who not only pursued his father but also set fire to the forest in which he and Hannah are now trapped.

But forest fires aren't exactly something you can generate willy-nilly. So, for a film in which hundreds of acres of land needed to be consumed by flames, director Taylor Sheridan sought the services of a visual effects team who could convincingly simulate the terrifying scale of a rapidly advancing natural disaster.

Making convincing computer-generated fire in movies is tricky, regardless of scale. “Historically it's been one of the more difficult things you can imagine to do in CG,” says Matthew Rouleau of Rodeo FX, the company hired to do the job.

Rodeo's work took around seven months in total and involved handling a staggering amount of data. Nathan Arbuckle, the film's visual effects lead, says that one of the chief difficulties is giving the fire a “sharp edge” while allowing it to move realistically and without breaking any data constraints.

Other filmmakers who have had to tangle with fire know that it is a challenging beast. Speaking to The Verge, Joseph Kosinski – who directed 2017’s Only the Brave, another film about firefighters – said: “Fire is so complex, and capturing that unpredictability and chaos that it has within it was a very hard thing to do.”

Rodeo, who have worked on Stranger Things and won three Emmys for their work on Game of Thrones, began by creating the bank of digital assets the team would need in order to make a forest. Before burning a forest using CG fire, they needed to create a CG forest to burn. This meant that the team were not constrained by needing to cut between real footage of a forest – the film did shoot in a real forest in New Mexico – and the digital forest that Rouleau created as environment supervisor. The work took around two months and involved creating not only countless trees but also trillions of pine needles to litter the forest floor.

While Rouleau was working on this, Arbuckle would do fire tests on Rouleau's assets. “It was my responsibility to burn the forest that Matthew was making,” as he puts it. As always, he looked to the reference points that the director and art director provided. “That's the best starting point you can have when you approach anything in our industry,” he says. There were major bushfires in Australia when Rodeo was working on the film, which meant that, in addition to the official references, images of flames engulfing large areas were not difficult to come by.

But, says Arbuckle, he learned that forest fires don't have one particular appearance. “They all look very different and it really comes down to a creative choice on how you want this one to look.” He watched one video of a forest fire where a temperature gauge onscreen indicated how hot the area was getting while the fire raged. A forest fire can act like a convection oven, he explains – the flames heat the entire area and dry it at the same time. The video demonstrated that the air becomes so hot that the trees that aren't even on fire yet can already be emitting smoke before the flames touch them. As soon as the flames do touch them, the trees erupt from the inside out. As Hannah says in the film: “That eats everything in its path.” This ferocity was something the team really wanted to play up, says Arbuckle. He and Rouleau also learned during their research that there are Australian trees whose sap, once heated, can “explode” and burst out of the tree.

In visual effects, fire is created using fluid simulations that for all intents and purposes have a mind of their own. “It's very interesting when you've talked to the visual effects artists who worked on it,” said Kosinski about Only the Brave. “You set up a situation, and then you basically ignite it, and you get what you get. And if you don't like the way it looks, you have to start it over and re-simulate it.”

The other thing the film called for was that the fire acted almost as though it had a mind of its own and was deliberately seeking people out. “The fire was a character and so it had behaviours the [filmmakers] wanted to try and emphasise,” Arbuckle says. This is an example, he says, of a situation in which the visual effects team aren't simply being called upon to simulate real life – they are working to the artistic specifications of a director.

Another of Arbuckle's reference points was the controlled, 100 per cent real fire that the practical effects team created on a set with 140 trees. This was a look they were able to use as reference for fires during the day and at night. Another facet that helped was that the actors and the set of fake trees were lit up by huge orange softboxes – lights commonly used on film sets. “It gave us really good raw material to work with,” he says. One of the contradictions of being a visual effects artist is knowing that, ideally, everything should be created without using visual effects if possible. “You always try to use as much as you can practically because that's always what's gonna sell whatever the effect is that you're trying to engineer,” Arbuckle says.

Whereas it would be more practical to light a real campfire than bother constructing a digital one, it was clearly safer to call on Rodeo for the film's gargantuan blazes. (Likewise, if a film calls for a character to catch fire, it will often be simpler for a stunt person to don a fire suit and take the paycheque.) When he created fire, it was important that Arbuckle made it look as though it was the result of burning wood. A gasoline fire burns differently to a wood fire, which will generate more and darker smoke, for example. He and his team also had to create a huge amount of ash for the night sequences – an element that viewers might simply assume is real.

One of the reasons it's painstaking to simulate fire for movies is because creating items that behave uniquely is draining for a computer's storage capacity. Initially, Rodeo tried burning trees in such a way that each tree emitted exactly the same kind of flames and smoke but this looked odd because each burning tree should behave differently to the next. (For visual effects Rodeo favors a software application called Houdini, which companies like Disney, Dreamworks and Pixar also use.) Arbuckle explains that one shot onscreen might comprise 300 small simulations. Each simulation could contain a gigabyte of data per frame. When the team needed to create fire both near the camera and far away from it, the most taxing task for their computers' hard drives, there was at least a terabyte of data for a brief moment on screen.

Where their work on a show like Game of Thrones was a way of igniting people's imagination and helping furnish worlds that have never existed, on Those Who Wish Me Dead Rodeo were conveying a real truth about the real world. They helped to communicate the message at the film's core: fire is a uniquely terrifying phenomenon that can tear through trees and people in the blink of an eye. While visual effects may be no match for the real thing, they can get pretty damn close.

Those Who Wish Me Dead is in cinemas now