Girl on a wire: Sandra Bullock talks about her new space drama, Gravity

SANDRA BULLOCK WAS pretty much done with acting before Gravity came along. After all, she had already won an Oscar for her role as Texan socialite Leigh Anne Tuohy in 2009's The Blind Side. She had made films that were both critically lauded and commercially successful, including The Heat, her female buddy-action-comedy with Melissa McCarthy, that has made more than US$220 million worldwide. She adopted a son, Louis, now three, with whom she was happily and quietly nesting in Austin, Texas.

But then there came the call from Alfonso Cuarón, the acclaimed Mexican filmmaker behind Y Tu Mamá También (2001), and Harry Potter and the Prisoner of Azkaban (2004). Bullock had always loved the way, she says, "he manages to distil everything down to one essence". If there was one director that Bullock would uproot her life for, it would be Cuarón.

The result is Gravity, a film set entirely in space, and almost entirely focused on the characters portrayed by Bullock and George Clooney. The actress says that even now, long after the film was completed, she finds it difficult to explain what it is about. "I would tell my friends, 'Don't ask me about it'," she says. "How do you explain it?"

On the face of it, Gravity, which was written jointly by Cuarón - who dreamed of becoming an astronaut as well as a director - and his son Jonás, appears to be a sci-fi thriller, but Bullock is adamant that it is not a typical science fiction film.

She plays Dr Ryan Stone, a medical engineer who trades in life at a hospital on earth to venture into space. On her first space shuttle mission, accompanied by veteran astronaut Matt Kowalsky (Clooney), the shuttle is destroyed by flying debris, leaving the duo, quite literally, lost in space.

Already dealing with a personal tragedy, Stone now has to find the will to make it home. Combine the story with a brilliantly realised backdrop of an endless universe, and you have a compelling, edge-of-your-seat story. There is some investigation into the notion of existential voids, and the visuals are splendid. But there is also a strong human angle: Gravity has an emotional resonance, and the way Bullock portrays her character's fear and fatigue is palpable and emotionally engaging.

"When Alfonso and I met we didn't talk about the technology, or the type of film it was going to be. We spoke about his reasons for writing it, his very personal reasons," Bullock says. "We talked about what he wanted to say emotionally, and it just moved me so much. Here was someone who seemed to believe the same things I believe, things you just can't explain or understand. I knew then that if he truly was this person and I didn't go on this journey I would have lost an opportunity. So I had to give it to myself," she says.

Bullock, who has also worked as a film producer, is one of Hollywood's most powerful and celebrated women. She initially appears intimidating, almost stern. She barely smiled during the press conference that preceded the interview. But later, sitting in a hotel in a form-fitting dark dress, she is more relaxed. She talks about spirituality and motherhood and is funny and self-deprecating. Not dissimilar, in fact, to the likable characters she's played in films such as Miss Congeniality and Speed.

Bullock says she had to take some emotional risks to make Gravity. She had to train hard for several months prior to the start of filming at Britain's Pinewood and Shepperton Studios, and continue every day for the three-and-a-half months that the movie was in production. She had to deal with being suspended by wires for hours at a time. She also needed to adapt her acting methods, as much of the filming was done in a series of black boxes in which she could only communicate with Cuarón by speaker.

Aside from the technical and physical aspects of her performance, Bullock also had to convey the deep and abiding pathos of a woman scarred by tragedy, trying to find her way home again in more ways than one. "Alfonso used the technology to make the movie more emotional," she says. To prepare for her time in space, she spoke to an astronaut about how the body holds itself in zero gravity.

"I love the unknown, the science of space. I know that we're not alone, and if we think we're the only ones around, we need to take a narcissist class. I'm curious to know what exists out there, but the idea of travelling just to hang out there ... I'm not brave enough. I still hate flying," she says, referring to her deep-seated fear of air travel.

There were more than a few moments of frustration, Bullock says. She would spend hours cooped up in what she described as a "lifeless chamber", waiting for the shot, and would often have to reshoot to get it exactly right. "Everything had to be so precise on this shoot that there was no room for any improvisation. Alfonso would say to me, 'Your nose was a quarter inch out of the frame, so we have to do another take.' It was very frustrating, but I understood it."

In an industry teeming with volatile, demanding personalities, Bullock remains known for her down-to-earth attitude. At 49 years of age, she remains a highly sought-after actress who has had a string of consistently successful movies since Speed shot her to stardom in 1994.

Born in Arlington, Virginia, she came from a talented family. Her father, John W. Bullock, was a voice coach, and her mother, the late Helga Meyer, was a German opera singer. Her father also served in the army, and between that and her mother's singing career, Bullock travelled extensively as a child, spending time in Salzburg and Nuremberg. She speaks fluent German and studied ballet, piano and the vocal arts as a child.

She has been involved in some celebrity scandal. Her five-year marriage to reality TV star Jesse James ended suddenly when news broke, shortly after she won the Oscar, that he had been repeatedly unfaithful. She went into hiding, and subsequently divorced James in 2010, adopting baby Louis as a single parent. Her career now seems stronger than ever.

Gravity provides a metaphor for her own life, where she cites her biggest personal priorities are "getting to the place where you can let go and have total acceptance". Much of that has to do with Louis, she claims. She took him on set with her while filming in Britain, ensuring that everything was baby-proofed (he was 18 months old at the time).

"Everything now is structured around my life with Louis," she says. "I think about whether I really want to do something before I drag him all over the world. But I also want him to have these amazing life experiences. He loves to explore other cultures and languages and people. And he's such a good traveller. I see how people look at us when we get onto a plane, me and a little kid, for a long flight, and I want to say to them, 'He'll be better behaved than you will be after a couple of cocktails!'"

Despite the intensity of filming Gravity, Bullock says she was able to decompress after a hard day's filming. She would meditate if the shoot got too stressful, and then dig deep into herself to find the emotion, the touchstone, with which she had to connect to get the shot just right.

"But after work, I could totally switch off," Bullock says. "Then Louis and I would just go off and have fun together."

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Gravity opens on October 3