The most certain thing about Gravity was always the headlines about it 'rocketing' and 'soaring' up the box office charts, revenue going 'out of this world' if it was to become a hit.

Much less certain was that Gravity itself would exist at all. After Universal Pictures passed on the project, Warner Brothers picked it up and planned to cast Robert Downey Jr and Angelina Jolie. But if the box office, critical and audience reaction has proven anything, it's that director Alfonso Cuaron's combination of George Clooney, Sandra Bullock, incredible visuals and edge-of-seat tension was the best possible choice.

One of thing that strikes cine-literate viewers during the awe-inspiring (and terrifying) exterior scenes is exactly how they're done. It's all so realistic you can easily picture Bullock and Clooney suspended in a sound stage on wires and gimbals, the Earth and spacecraft animated in later. In fact every scene of space exteriors is CGI, the actor's faces animated onto digital backgrounds of spacecraft, the bodies of the astronauts and the nearby Earth.

So how does a production designer – a filmmaking craft traditionally concerned with overseeing the designing and building of props, sets and locations – deal with a CGI production?

The answer, according to Gravity production designer Andy Nicholson, is a huge amount of preparation and then trust. According to the British-born artist, he knew going in it was going to be 'radically different from every other movie' because of the workflow between his department and the computer animators who'd put the exterior sequences together. "Instead of giving designs to draftsmen and construction teams we're giving it to 3D modelers, texture and lighting animators," he says.

The process began with as much grounding in reality as Nicholson and his team could manage. Even though no real parachute, escape hatch or spacesuit helmet would appear on screen, the initial stages were all about collecting real materials in order to give animators the best possible reference about how certain surfaces would behave.

"We put the word out to let us know about anything they were having trouble animating," he says. "Because the modeling department was making principal interiors we could make a piece of a solar panel – we knew we could get a piece of what they're made of and make a piece of one so the animators had the real thing and could see how it moved and reacted to light."

Luckily, a large amount of what they had to research was in the public domain. Nicholson says his team worked with literally thousands of photos, and because – for example – the International Space Station has been built and added to steadily over the years, they gave the animators very detailed images.

Materials to make pieces of spacecraft or equipment were sourced from everywhere, even enthusiast websites. Pieces of rubber from the shuttle's tyres, spacesuit faceplates – it didn't matter. Nicholson just knew it all had to be real.

So why the realism when the pictures on screen were going to essentially be cartoons? "It had to be real," Nicholson explains, "or it was going to fall apart."

Getting it moving

The next step was designing each shot with director Alfonso Cuaron, which meant digitally sculpting people and spaceships and sending the final models to animators to add lighting effects, surfaces and every other aspect of falsified reality. Once 'spatial arrangement' of the shot had been figured out, the pre-visualisation sequence would assemble it into a rudimentary version of the final shot.

After watching the pre-viz shot, Cuaron would then send notes back to Nicholson – anything from the location of a module on a ship or the number of hand holds for the digital actors to grab to a hatch needing to be bigger or smaller to suit the story (the Russian Soyuz doesn't actually have a hatch and window on the side as depicted in the film).

Nicholson would rework the basic models to incorporate the changes and send it back to the animators, who'd jump back into Maya and Cinema4D to rework the pre-viz sequence. "We'd often go around five or six times tweaking a shot and sending it back," he says.

So where did trust come into it? The modeling and concepting done by Nicholson's team was all early-stage production. Most of the animating was done according to his work in the 6-9 month after his part in Gravity was already over, which meant anything they sent to the animators had to be rock solid.