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Bullock called space, Clooney did yoga for 'Gravity'

US actress Sandra Bullock said on Wednesday she spoke to astronauts to prepare for the harrowing space drama "Gravity", while co-star George Clooney quipped he just used yoga and booze.

"I called and left a message and they called me back" from the International Space Station, a toned Bullock said at the Venice film festival after a press screening of the film.

"They were incredibly helpful," she said.

"They gave me an inside visual as to why they chose what they do for a living, their love for what is beyond our planet."

The 3-D sci-fi thriller sees Clooney and Bullock as astronauts who are flung into dark, deep space when a debris shower destroys their shuttle.

"It was the craziest, most bizarre challenging shoot I've ever done," said the actress, who explained she had also worked out intensely and listened to opera music to prepare for her part.

Clooney, dashing in a light grey suit, joked that he had "done yoga and drunk my way into the part" and said the biggest challenge was learning how to "move slowly to mimic body movement in space while speaking quickly".

Directed by Mexico's Alfonso Cuaron of "Children of Men" fame, the film induces nail-biting anxiety, with terrifying shots from inside the astronauts' helmets as they spin wildly and lose all radio contact with Earth.

"We wanted a film about adversity, and used the debris as a metaphor," Cuaron said.

"The idea was for a film in which we could strip the narrative down as much as possible... a story of just two characters in a very hostile environment who undergo a journey, a metaphor the audience could connect to," he said.

Cuaron invented new filmmaking techniques to depict spacewalking, including shooting inside a giant cube to evoke constantly shifting light sources.

"We had advisors, scientists and physicists teaching the cast how things would react in space. A lot of the shots required the actors to be isolated, it was a very abstract way for them to perform," he said.

After months of delay and a huge budget, "Gravity" delivers a Hollywood punch.

Bullock plays Dr. Ryan Stone, a medical engineer on her first mission who relies on veteran astronaut Matt Kowalsky (Clooney) to hold on to her sanity and try to survive despite her rapidly dropping oxygen levels.

A soundtrack dominated by her racing heartbeat and the deafening silence of space is punctuated by jokes from Kowalsky: "Half of North America just lost its Facebook", he cracks as debris takes out communication satellites.

Stone spends much of the film alone, and Bullock said it was not easy.

"I had to create an inner life. I was alone much of the time, and if I had just a glimpse of George while hanging from 20 foot cables I was grateful," she said.

"I was very grateful for any human contact, even someone's breath."

The film's key message, Clooney said, was about how mammoth a task it can be to "come through adversity and end up with your feet planted on the ground" -- a reference to the safe landing on Earth the characters long for desperately.

While director of photography Emmanuel Lubezki produces stunning images which left the festival spectators gasping for air, the humour sometimes detracts from key scenes and the characters have disappointingly little time to develop.

Clooney, Bullock and Cuaron will take to the red carpet for the opening ceremony of the world's oldest film festival, followed by an exclusive after-dinner party later on Wednesday.