When you're shooting a movie called "Gravity," your production has one big problem to overcome.

Gravity.

"That was the biggest obstacle," said director Alfonso Cuaron last month at the Toronto International Film Festival. "There's no up, no down, and everything is in constant motion. How were we going to shoot it? There was no technology to do it, so we had to invent new tools."

Stars Sandra Bullock and George Clooney, their faces covered by masks and their bodies in a constant float, also had to devise a new way of acting.

"It was more like being a part of Cirque du Soleil than what we're used to as actors," Bullock said, explaing that sometimes she would have to perform on a spinning bicycle seat or office chair. "It was very frustrating. But I loved it."

"Gravity," opening today, is the story of two astronauts struggling to stay alive after they become untethered from their spacecraft.

The idea for the film started with Cuaron's son, Jonas Cuaron, also a filmmaker. But even with the elder Cuaron's pedigree ("Y Tu Mama Tambien," "Harry Potter and the Prisoner of Azkaban," "Children of Men"), the film took 4 1/2 years of planning before the Cuarons were ready to begin serious conversations with actors. About two years ago they met with Bullock - who was not their first choice and is well aware of it, she joked.

"But I always feel the right person ends up making the film," she said seriously.

Once Bullock was cast, "We shaped the whole thing to Sandra's voice," Alfonso Cuaron said. "Her discipline is scary."

Bullock, who took the role even though she's afraid of flying ("I appreciate not being in my comfort zone," she said), originally was told the movie was going to be shot in the "Vomit Comet," the zero-gravity plane used by NASA. That had her freaking out through most of her preparation.

Although the filmmakers decided against using the "VC," producer David Heyman didn't tell her they'd changed their minds until just a week before shooting.

An odd grin on her face, Bullock said she hadn't learned that until that minute.

Bullock said the shoot was still incredibly grueling, explaining she had trained with dancers to perfect her body movement and had to be put back together at the end of the day by physio-therapists.

In keeping with the role, Bullock said she also tried to "remove as much as I could of her [character's] womanhood."

"It was lonely," she said. "I missed the sun. I missed being with my son. Emotionally it was the Wild West, and I tried not to take it out on Alfonso."

The director understood, but he had his own problems.

After consulting with astronauts, he "realized we were morons" when it came to gravity. What they hadn't taken into account was recoil. In space, if you throw a ball, that ball will go until something gets in its way to stop it. In the same way, the motion of the arm needed to propel the ball forward will send the body backward, and it will go until something gets in its way.

So all the movement had to be counteracted.

"We didn't know if it was going to work," Cuaron said.

Warner Bros., however, supported the movie from the beginning and even gave the filmmakers an extra year when they asked for it.

Cuaron said that the process mirrored the line often repeated in the movie, "Houston in the blind," which the astronauts repeat into their radios to a mission control with whom they've lost contact.

For the studio, Cuaron said, "this was like financing a fairly big movie in the blind."

As for co-star Clooney, who was unable to attend the Toronto fest this year, Bullock said, "George and I have known each other over 20 years [before either was successful]. He's the same exact person I knew then in all respects. He's taken his blessed life and done amazing things."

Regarding an actual trip to space, Bullock isn't ready to sign up . . . yet.

"I have no desire to go," she said. "Maybe when I'm 80, if my son wants to go to space with me, and I feel like I did a good job . . ."