"In some ways, it was doing things backwards and then forwards again," Webber elaborated. "So we had to finesse the animation to quite a large degree before we started shooting it. Having said that, we also had to factor in flexibility because you're working with actors, so when you have shots that go on for a minute and there are long dialogue passages, you need to make sure there is room for the actors to breathe. Certain sections required adapting live to how the performance was going."

For example, Webber convinced Cuaron that it would be better to adjust how Bullock moves through a fire that erupts aboard the ISS. The explanation of how he was going to work it out with Framestore was complicated, but it enhanced the performance.

Even after they were nearly finished, the director realized that the opening shot of the space shuttle would look better if it was upside down. But it took another two months of animated tweaking. "They had to slightly re-animate and then completely re-render a long section as it turns the right way up because it goes through a spin," Webber recalled. "And some extra modeling went into revealing more clearly other parts of the shuttle and Hubble that had not been seen close-up. But it was worth making a change: It was a better view of the Earth and more disconcerting seeing it upside down and than right side up."

Yet Lubezki needed a way of seamlessly integrating live action and CG as well as real and virtual lighting to believably simulate zero gravity. He found his answer in what is already being referred to as the legendary "Light Box," which Cuaron termed "a monologue inside cinema." Also, for the first time, Framestore used the physically-based rendering system known as Arnold to attain such realistic-looking and beautiful imagery.

And they relied on Webber to make the 10-foot x 20-foot box work: "It gave us huge flexibility with the LED lighting," he observed. "To spin it around the character was hard enough, but it also had to change color and required detail in the light to get the right surface texture, to get the right lighting on her face. When Sandra's supposed to be isolated in space, and she finds herself isolated in this box, it worked quite well. And when she was spinning away and shut in this box and could see a massive picture of the Earth, it gave her a guide for what was going on around her."

"Gravity" additionally required a totally different mindset from the animators, who were used to dealing with weight. "We retrained them about what goes on in space and they had to break their habit of animating in certain ways," Webber continued. "It was unnatural at first. We also used some simulation and other tricks just to see what would happen if someone got thrown against the space ship and bounced off or if someone's being tugged along by someone else with their safety tether. You would discover unexpected results and that provided good ideas for bouncing around and how hard it was for George to control Sandra by his tether."

The detail of the animation is absolutely stunning. There's a new kind of cinematic intimacy being inside Bullock's helmet with her, watching the panic-stricken breaths she takes and seeing the reflection of the shuttle in her eye.

Or the mundane moments we're not supposed to notice, such as when Bullock fixes the electronics on the Hubble telescope, turning knobs and pushing circuit boards. There's a subtlety required in the gloves and the way the boards bounce and stick as she pushes them in and the tethers getting knocked out of the way.

By contrast, Bullock's falling tear drop becomes larger than life and is the best 3-D effect in the entire movie (and was scripted to be so).

"Gravity," therefore, represents yet another milestone in virtual production. There are no CG characters, but it's "Life of Pi" in space with Clooney as the tiger. Cuaron delighted in the freedom to be metaphorical about adversity, survival, and rebirth and playing with such iconography as the fetus, the womb, and the umbilical chord. 

"But without the performances -- one more cinematic tool -- you don't achieve the emotional experience that you're after," the director cautioned.

Ultimately, that's what pure cinema is really all about.