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When The Aeronauts was screened at the Toronto International Film Festival in September, director Tom Harper told the audience that he was “really struck by the thirst for adventure and the extreme risk” the film’s leading characters take. The same could be said about Harper for undertaking such an ambitious project, most of which takes place in the basket of a hot air balloon high above the earth.

Set in London in 1862, The Aeronauts follows the death-defying attempts of real-life pioneering meteorologist James Glaisher (Eddie Redmayne) and his gutsy balloon pilot Amelia Wren, a fictional composite (Felicity Jones), to fly higher than anyone in history.

The film marks the on-screen reunion between Eddie Redmayne and Felicity Jones who costarred in The Theory of Everything (2014), which earned Redmayne an Academy Award for Best Actor. Jones’s performance in The Aeronauts has already earned her Oscar buzz, with The Hollywood Reporter‘s awards columnist Scott Feinberg including her in a list of “Major Threats” in the Best Actress category.

Amazon Studios had initially planned on a full theatrical run for The Aeronauts, which reportedly cost more than $50 million to make. But after several of Amazon’s film acquisitions, including Sundance comedies Brittany Runs a Marathon and Late Night, underperformed in theaters, the theatrical window has been narrowed to two weeks. Following a limited theatrical run beginning Dec. 6, the film will debut worldwide on Amazon’s Prime Video on Dec. 20.

Fortune chatted with The Aeronauts director Tom Harper at the SCAD Savannah Film Festival, where the film screened in late October. Until now, the British film and television director has been best known for his work on the 2016 BBC mini-series War and Peace and the 2018 musical drama Wild Rose.

This conversation has been edited and condensed for clarity.

How did this project come about?

While we were making War and Peace in Russia, my cinematographer, George Steel, who I’ve worked with for many years, said he had just heard about a book that’s about to come out involving these people who go on this flight to 36,000 feet. “Wouldn’t that be an amazing thing to make a film entirely set in the sky?” he asked. “Imagine the cinematographic opportunities!”

I bought the book [Richard Holmes’s Falling Upwards] and it was a remarkable account of how these two people went to 36,000 feet, which is as high as a commercial airline flies now. I thought that was an incredible feat, but not enough to make a film in itself. The scientist and the pilot didn’t really speak at all during the whole flight because they were taking measurements, which was admirable, but doesn’t make for the best cinematic experience necessarily. But then I read some of the other stories and about some of the other characters and then I suddenly thought, “what if we create an amalgam?”

I started putting together a story based on a variety of these characters. I took it to my friend and longtime collaborator Jack Thorne, who’s a brilliant screenwriter. We’ve made five projects together. He wrote the script and away we went!

When did Amazon come on board?

We wrote the script “on spec” and took it to producers David Hoberman and Todd Lieberman at Mandeville Films. They came on board. They had just come off of producing [the 2017 live-action] Beauty and the Beast. We knew we weren’t going to be able to achieve this level of visual effects without some very experienced producers.

We took it to a number of different people at the same time and Amazon loved the project and they came on board at that point. Then it took another year for them to green light the project. From inception to greenlight, it was about two years and then it took another year to complete. That’s pretty speedy for a feature film.

Where did you film? I imagine the logistics were quite challenging.

We did all the main unit photography in the U.K., but it was always our goal from the very beginning to shoot as much of it for real as possible. Some of it is obviously not possible to shoot what’s real, like when they go through the storm. We could do some of it for real. So we built this enormous 19th century gas balloon and we put our actors in it and up they went. We used cameras in helicopters and drones and in the basket of the balloon. As you can imagine, logistically, it was quite a challenge. Just getting anywhere to fly, let alone the perfect clouds.

How do you go about finding the perfect clouds?

The chances are, if you’ve got the perfect storm clouds you want, it’s not a good day for flying. So we sort of went cloud-chasing all around the world. We went to New Orleans. We went to South Africa. We went all around the U.K. trying to get these perfect cloud formations. In pursuit of reality, the vast majority of the skies you see are real skies where we filmed wherever we could get the best cloud formations.

'The Aeronauts' U.K. Premiere - 63rd BFI London Film Festival — Felicity Jones, Tom Harper, and Eddie Redmayne attend “The Aeronauts” U.K. premiere during the 63rd BFI London Film Festival at the Odeon Luxe Leicester Square on Oct. 7 in London.

Did you always have Eddie Redmayne and Felicity Jones in mind as the leads?

We had our James Glaisher list and our Amelia Wren list and the top of the James list was Eddie and the top of the Amelia list was Felicity. We were like “Oh, well, that’s probably problematic because they just worked with each other on The Theory of Everything. Are they going to work with each other again so soon?” It seemed unimaginative. Then we thought, “well, that shouldn’t stop us from offering it the best people.”

They clearly have great chemistry. There’s a Hollywood tradition with actors working together with each other again and again. We sent the script to both of them at exactly the same time with the expectation and hope that they would communicate amongst themselves, which they did. And they loved working with each other last time and they said,”yes.”

It was actually a remarkably straightforward process and for me, a really lovely thing. So much of the director-actor relationship and the actor-actor relationship is about trust and about a space in which we can take risks and inspire each other and push each other and dare each other. That’s a very exposing thing. Because they have that bond already, it meant that they together already had a head start and they were very generous about including me in that. I mean the whole film is set in a basket, 8 foot by 8 foot. If they had not liked each other, it would have been problematic.

For a film that’s so cinematic, how do you feel about the fact that most people are going to see the film at home? I imagine there are pros and cons of working with Amazon.

Scorsese had it right when he said about Netflix and The Irishman, “They gave me the money I needed to make the movie.” I really feel that. Amazon backed it from the very beginning and gave us the chance to make this film. It’s an original film. It’s not based on an existing intellectual property. It’s a risky prospect in many ways. When it comes to how it’s seen, it is cinematic and it was always designed to be cinematic.

Do you have a preference for how people see the film?

I do hope that people go and see it in cinemas. I think that’s a great way to see the film, but I’m also grateful that it’s going to be seen by millions of people all around the world. I think people will have a choice and I just hope that people see it in whatever form they can really.

How did you manage the logistics of the hot-air balloon sequences?

It was a nightmare. We basically did it in two different ways—one is for real in a balloon with a helicopter and that was logistically, extremely challenging. Ironically, in a film about weather, it was all based upon weather. You can only fly a balloon—particularly with your two very precious leading actors—under five knots—so you’re constantly waiting. So everyone’s on standby waiting for the right weather.

That could get very expensive if you’ve got your location staff and your stunt team and your helicopter flight unit and your special aerial cameras. So that was really challenging. There’s one stunt in the film that we did for real—where Felicity climbs up the side of the balloon. That was a real stunt person—there was a ton of preparation involved.

Then working in the studio—that was equally challenging. How do you get an 80 foot balloon into a studio and how do you chop it into bits so you can film it in sections? I think with less brilliant people and with less “up for it” cast—they did most of their own stunts—it would have been impossible.

In terms of visual effects, was a lot done in post-production?

There were a lot of visual effects, obviously, but not complete visual effects. Most of what we shot was real, but the visual effects part of it is putting it together. So to give you an example, we did shoot Felicity Jones sitting on the edge of the basket 3,000 feet above London, but London back then, in 1862, is not like London now, so there are loads of visual effects there, but it doesn’t mean people didn’t do it for real. The backgrounds are real—they’re the clouds we shot in South Africa or New Orleans and the foreground is real. Every shot is different in how it’s constructed, but it’s all sort of real.

‘The Aeronauts’ Director on How He Pulled Off the High-Flying Adventure Film

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