Sandra Bullock says her training as a dancer and carrying around her 40-pound three-year-old helped her to get into top shape to play an astronaut drifting through space in Gravity.
The actress plays a scientist stranded hundreds of miles above Earth with a wise-cracking astronaut George Clooney when debris destroys their space shuttle.
She said on Wednesday, before the film's world premiere at the Venice Film Festival, that her character had given up on life because of a tragedy, and she wanted to express the physical changes brought by enormous loss.
The star said "I wanted her to lose everything feminine or maternal."
Her co-star deftly deferred a question over possible US intervention in Syria, saying "I actually thought you would ask me about Ben Affleck playing Batman. But no, it's Syria."
The stars arrived in Venice by motorboat to kick-off the 70th edition of the film festival with Gravity, the 3-D Alfonso Cuaron film, which opens the festival out of competition on Wednesday.
Twenty films are vying for the coveted Golden Lion at the world's oldest film festival, which closes on September 7.
The jury is headed by Oscar-winning director Bernardo Bertolucci.
Golden Lion contenders include Terry Gilliam's The Zero Theorem, Peter Landesman's Parkland and Kelly Reichardt's Night Moves.