When Alfonso Cuaron was first planning his marooned-in-space drama “Gravity,” the filmmaker imagined an action-drama set in orbit, most of it with just a single character who would be weightless for the entire picture. It would be filmed using his signature long, continuous shots.

But there was one big problem: As fellow director David Fincher warned Cuaron and his cinematographer, Emmanuel “Chivo” Lubezki, the technology to make that movie simply didn’t exist yet. He advised them to wait five years.

“We were stubborn, (and) said we’re going to make it work,” Cuaron tells Variety. “But you know what? David was right. It took us 4½ years.”

“Gravity,” which opened the Venice Film Festival to great fanfare — and early Oscar buzz — and will screen at Telluride and Toronto on its way to an Oct. 4 release, is as much of a game-changer, in its way, as “Avatar” was. Perhaps that’s fitting, since “Avatar” director James Cameron, whose pal Cuaron screened the fi lm for him four weeks ago, is among the picture’s most ardent champions.

“I was stunned, absolutely floored,” he says. “I think it’s the best space photography ever done, I think it’s the best space film ever done, and it’s the movie I’ve been hungry to see for an awful long time.”

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Cameron, surprisingly, said it wasn’t the cutting edge technology that impressed him; it was Cuaron’s uncompromising vision and tenacity in getting the picture made as he wanted it, and star Sandra Bullock’s preparation for and performance of a highly challenging role. “What is interesting is the human dimension,” Cameron says. “Alfonso and Sandra working together to create an absolutely seamless portrayal of a woman fighting for her life in zero gravity.”

The 91-minute picture — unusually short in today’s world of two-hour-plus tentpoles — follows two spacewalking astronauts, commander Matt Kowalsky (George Clooney) and mission specialist Ryan Stone (Bullock), whose journey becomes a fight for survival when a fusillade of debris cripples their shuttle, kills a shipmate and leaves them stranded in space. As they make their way to the Intl. Space Station and beyond, searching for a lifeboat, it falls to Stone, on her first space expedition, to find a way back to Earth despite a terrifying series of setbacks.

A lot is riding on “Gravity” for Bullock, the filmmakers and their studio backer, Warner Bros., which bankrolled the risky mission at approximately $100 million. For producer David Heyman, it is the first picture he’s steered since the blockbuster “Harry Potter” series. For Cuaron, it’s his first since 2006’s sci-fi thriller “Children of Men,” and by far his most challenging outing. For Bullock, it’s the first time the Oscar winner has toplined a CG-heavy sci-fier, let alone carried an entire picture on her own for most of its length.

Even with so much at stake, though, Cuaron stayed true to form, eschewing a tried-and-true approach to filmmaking in favor of breaking creative and technological barriers to bring a highly emotional story to the fore.

Photo by Julian Broad

“Alfonso does not play it safe,” says Heyman, who worked with the director on the third “Harry Potter” film, “Prisoner of Azkaban,” the darkest, and according to many the most daring, of the series. In “Gravity,” Cuaron and his team tackled unproven techniques and digital technologies aimed at transporting audiences into weightless space.

“There was no way to rely on anything we knew before this film,” Bullock says. “No character was like Stone, no film set was ever like these sets, not one member of this crew had ever done this before. We all were doing something that had never been done before.”

“Gravity” began to tug at Cuaron after his screenwriter son Jonas asked him for notes on a struggle-for-survival script he was writing, “Desierto.” The elder Cuaron had a few notes, but liked the script’s stripped-down narrative.

“What I really wanted to do is something in which, in a single through -line, you can play and juggle with different things and motifs and serious subject matter without stopping the action,” Cuaron says. “I said (to Jonas), ‘I would love you to help me do something like that,’ and he got excited (and said), ‘OK, let’s do something together.’ ”

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They soon settled on a theme they wanted to explore: “Adversity and the possibility of rebirth as an outcome of adversities,” explains the director.

Cuaron, a space buff from the days of the Apollo moon shots, proposed they set the story in space, in the present day. They focused on one of NASA’s real-life nightmare scenarios: the Kessler Syndrome.

In 1978, NASA scientist Donald J. Kessler theorized that there are enough objects in low-Earth orbit that a collision between objects could have a cascade effect, with the debris from the first crash causing more crashes, which shoot off more debris, which cause more crashes, and so on. Once the Kessler Syndrome starts, anything or anyone in orbit would be subject to a lethal bombardment by shrapnel moving at tremendous speeds. “We decided to take that as a metaphor for adversity,” Cuaron says.

The Cuarons developed the script at Universal, where Alfonso had directed “Children of Men” in 2006. Universal was hoping to attach Angelina Jolie to the project, but decided the picture would be too expensive, and put “Gravity” into turnaround. Warners picked it up, and Cuaron cast Bullock and Robert Downey Jr. in late 2010 for the leads. Downey subsequently dropped out and was replaced by Clooney at the end of the year.

Photo by Julian Broad

Meanwhile, the director and his team worked on developing a new way of shooting to go with their script.

There had been realistic space pictures before, notably Ron Howard’s 1995 hit “Apollo 13,” but they’d used a combination of actors on wires in front of bluescreens and short takes of weightlessness shot on NASA’s Reduced Gravity Aircraft, a.k.a. the “Vomit Comet.” That wouldn’t work for Cuaron’s style, with its extended takes.

Early in development, Cuaron contacted his longtime friend and cinematographer Lubezki. The pair had evolved their long-take technique, which sometimes covers an entire scene in a single shot, starting on “Y tu mama tambien,” released in 2001.

“When (Cuaron) called and said, ‘I want to do a movie in space,’ immediately I started seeing the movie with all these very long shots I’ve never seen before, especially in a science-fiction movie,” Lubezki says. “But I got a chill because I realized how hard it was going to be to do something like that, when you have an actor in zero gravity and you’re not cutting.”

For some scenes where they’d be seated inside space capsules, the actors playing Stone and Kowalsky could be shot in real, physical sets. But most of the movie had one or both of them floating free, either in a spacewalk or inside a space station. How to shoot such scenes? And on top of all that, Cuaron wanted the picture to be in 3D.

The director and cinematographer met with a number of friends and tech-savvy directors, including Fincher and Cameron. “I read the script, and I thought it was tremendously challenging to shoot with a high degree of veracity to get the real look of zero gravity,” Cameron says. He recommended some performance-capture techniques he’d used on “Avatar,” but Cuaron opted for a different route.

SEE ALSO: Venice: Bullock, Clooney Launch ‘Gravity’ at Festival

Adds Cameron: “I’m sure Alfonso had a real uphill battle with the studio, with everyone involved, to get it the way it needed to look. But he knew in his mind how it needed to look, and he went after it.”

Cuaron enlisted visual effects supervisor Tim Webber, with whom he had also collaborated on “Children of Men.” “We said if somebody can do it, it’s Tim,” Lubezki recalls. The trio of Cuaron, Lubezki and Webber soon concluded they couldn’t make “Gravity” as they wanted to by simply using traditional methods. So for the spacewalk scenes, says Webber, “We decided to shoot (the actors’) faces and create everything else digitally. Which was quite a difficult decision.”

To do that, Lubezki reasoned, he would need to light the faces to match the all-digital environment they’d be put into. Whether the characters were floating gently, changing direction or tumbling in space, the facial light would have to be a perfect match for the Earth, sun and stars in the background. “That can break easily,” explains Lubezki, “if the light is not moving at the speed that it has to move, if the position of the light is not right, if the contrast or density on the faces is wrong, et cetera.”

Photo by Julian Broad

Lubezki suggested folding an LED screen into a box, putting the actor inside, and using the light from the screen to light the actor. That way, instead of moving either Bullock or Clooney in the middle of static lights, the projected image could move while they stayed still and safe. This “light box” became the key to the spacewalk scenes. But it was only a nine-foot cube, just big enough for one actor, not an actor and a camera crew.

Another puzzle remained. Some scenes would show Bullock’s Stone floating weightless inside the space station. If she appeared to be straining, that would break the illusion of weightlessness. So the filmmakers turned to industrial robots: specifically, robots designed for automobile manufacturing. They could carry heavy loads — the full weight of an actor — and move as quickly or slowly as required. Importantly, they could operate in less space than a cameraman or grip, and are incredibly precise.

However, they must be programmed well in advance. So the entire picture would have to be pre-planned and pre-visualized in fine detail: all of the blocking for the actors and the camera, every angle, every zoom, all the digital sets and backgrounds, the color and position of every light source.

“Pretty much we had to finish post-production before we could even start pre-production, because of all the programming,” Cuaron says.

SEE ALSO: Venice Film Review: ‘Gravity’

For interiors where Bullock would be out of her space suit, the filmmakers designed rigs to support her and move her around. She’d appear to be pushing herself along, but the robot would do all the work. “Luckily,” Webber says, “Sandra was incredibly physically adept and very good at doing perfectly timed graceful movements while hitting complicated marks in 3D space and giving a great performance at the same time.”

With so much new technology in play, the filmmakers redoubled their efforts to make sure the whole project didn’t devolve into a technical exercise. Cuaron insisted Lubezki light the entire picture, not turn the lighting over to the vfx team. Lubezki spent months working with animators and digital lighting directors (he calls them his gaffers) at Framestore in London.

“I would wake up at 4 a.m., turn on my computer, I’d say good morning to my gaffer and start working on a scene,” Lubezki says. “I would say, ‘Move the sun 60,000 kilometers to the north.’ That way I could put the lighting anywhere I wanted.”

Sound design and the score, too, would play a critical role. There is no sound in space, so much of the action, even the debris storms, plays out against eerie silence, broken only by the score. The silence is more startling after the score builds to deafening crescendos, then stops abruptly. But during the interior scenes, the rumbles and groans of failing space gear are as frightening as the roars of any classic movie monster — even more so because their source is unseen.

The most human part of the process couldn’t begin without the cast, though. Jolie and Natalie Portman were each offered the role of Stone before Bullock came onboard after winning the Oscar for “The Blind Side” earlier that year. The actress spent six months in physical training to prepare for shooting while reviewing the script with Cuaron in meticulous detail.

Says Cuaron, “More than anything else, we were just talking about the thematic element of the film, the possibility of rebirth after adversity.” They worked out how she would perform each scene, and her notes were included the pre-vis animation and programming for the robots.

Cuaron and Bullock zeroed in on one crucial aspect of the performance: Stone’s breath, “and how that breath was going to dictate her emotions,” he says. “That breath that is connected with stress in some instances, but also the breath that is dictated by lack of oxygen.”

Their conversations covered every detail of the script and Bullock’s character. “She was involved so closely in every single decision throughout the whole thing,” Cuaron says. “And it was a good thing, because once we started prepping for the shoot, it was almost more like a dance routine, where it was one-two-three left, left, four-five-six then on the right. She was amazing about the blocking and the rehearsal of that. So when we were shooting, everything was just about truthfulness and emotion.”

Cameron says Bullock’s work is more impressive than the technology that supported it. “She’s the one that had to take on this unbelievable challenge to perform it. (It was) probably no less demanding than a Cirque du Soleil performer, from what I can see.” And of the result, he says, “There’s an art to that, to creating moments that seem spontaneous but are very highly rehearsed and choreographed. Not too many people can do it. … I think it’s really important for people in Hollywood to understand what was accomplished here.”

The set Bullock walked onto didn’t resemble a normal movie shoot. At one end of the stage was the light box, with a small hole in one side. Outside that hole was a track extending away from the box, and on that track, the robot holding the camera. Inside the box was a rig Bullock would be strapped into. In the wings were rows of computer workstations with technicians controlling the light box, lighting, camera and the robots.

“In the end, all the homework we did had to be let go once I stepped into the apparatus,” Bullock says. “You had to trust you knew who the character was at that point and just mentally wrap your head around a most unnatural setting for an actor … and make it natural.”

Because it was laborious to get in and out of her rig, Bullock chose to stay inside the light box alone for nine or 10 hours at a time, communicating only through a headset. Though she calls those hours isolating and silent, she adds, “It also gave me the opportunity to dig as deeply as I needed to for whatever was required, in privacy. … To me it felt as though there was nothing but the thoughts in my head to give me company.”

With such an odd shooting space, and with only one actor on set much of the time, Cuaron says his biggest challenge was how to keep a warm, friendly set, “a place that is fun to play in,” he says. “Not to (have it) become a technical game, but all the time to keep the creative aspect in the forefront of everything.”

When Bullock arrived each day, there would be a mass celebration, including a “Rocky”-like fanfare and a big lighted sign atop the light box that read “Sandy’s Cage.”

Once in the box, Bullock knew what parts of her performance she could and could not change, while Lubezki and Webber were careful to give her some lattitude. “(Lubezki) being an amazing cinematographer,” Cuaron says, “he’s always very concerned about the atmosphere on the set and the atmosphere for the actors.”

Lubezki remembers many days on the shoot being “scary,” because some of the bits of gear simply wouldn’t work sometimes. Adds Cuaron, “You don’t know if it’s just a simple cable or if it’s something that has to do with software.”

Lubezki often worried that the gear would break down and shooting would have to stop for days or weeks.

“He’s always scared to death,” says Cuaron with a laugh, but the director concedes his cinematographer had a legitimate concern. The day-cost for the shoot was on par with a typical tentpole, around $150,000, but that didn’t include planning and programming costs.

Worse, Cuaron says, the pipeline for the picture, from previsualization to shooting to visual effects, was delicate. “If something fundamental in the technology that we were doing had collapsed, it would mean that the film would collapse. That was the spookiest thing. Any collapse in the pipeline would have huge ripple effects. So there were moments in which we felt the whole film had collapsed.” In those moments, Clooney and Bullock worked hard to keep spirits up. But in the end, Cuaron says, their rigs were never down for more than about six hours at a time.

In the end, those behind the scenes agreed that the technology and prep succeeded in one crucial respect: It let the filmmakers — and the movie — zero in on Bullock’s performance, which is already being buzzed about.

“The shoot for me was those Eureka moments,” says Cuaron, recalling one particularly absorbing closeup of Bullock as Stone when the character is talking about her daughter. “You’re cool that the light actually worked this time?” he asked Lubezki. “Was everything in sync?” he queried Webber.

“The two of them said, ‘Of course. Forget about that. Do you see what just happened?’ Suddenly, with all that weight of technology, we were capturing a great performance. … It was those moments in which everything came together for what was the point of this film, the experience of a human up there.”

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Helmer’s TV debut

Even while juggling “Gravity,” Cuaron has made his first foray into TV series production in partnership with Warner Bros. and J. J. Abrams’ Bad Robot banner. “Believe,” a fantasy drama about an orphaned girl with supernatural powers, is set for a midseason bow on NBC. Cuaron co-created the series and directed the pilot last fall in New York. He will continue to have a hand in the show’s creative direction through his role as exec producer.