A dozen major actresses would have leapt to take her place, but Tarantino refused to dump Thurman - not only because they are longtime pals, but also because he and Thurman came up with the idea of "Kill Bill" together while working on "Pulp Fiction."
"There was no question," says Tarantino of recasting his movie, which opens Friday. "It was like, would Sergio Leone have replaced Eastwood in 'A Fistful of Dollars'?"
On Jan. 15, 2002, Thurman gave birth to Roan, her second child with now-estranged husband Ethan Hawke (search) - and just two months later, she left her home in New York City and reported for duty at the "Kill Bill" training center, a converted warehouse in Culver City, just south of Los Angeles.
"I was so out of shape it was ridiculous," recalls Thurman, who had gained 50 pounds during her pregnancy.
She faced three months of training before she had to go before cameras as the butt-kicking character The Bride, who leaps off 20-foot high ledges and savages Japanese assassins with her samurai blade.
But the first time Thurman picked up one of the 10-pound swords, she swung it around and accidentally smacked herself in the head - nearly bursting into tears.
Tarantino told her to suck it up because he was determined she do most of her own stunts.
"There was no way we were going to use quick cuts or CGI [computer graphics]," he says. "Not in this movie."
Thurman trained at least eight hours a day, five days a week, and not doing her favorite exercise, yoga.
"They said, 'Forget it - it's weightlifting for you,'" Thurman recalls.
Then came the fight training, with Hong Kong director and fight choreographer Yuen Woo-ping, of "Matrix" (search) and "Crouching Tiger, Hidden Dragon" fame. He drilled Thurman in three styles of kung fu, two styles of sword fighting, plus knife throwing, knife fighting and hand-to-hand combat.
"The first week or so I ended up in the bathroom crying to myself," admits Thurman, 33, who brought Roan and her first child, 5-year-old Maya, to the California training center.
"I'd come home to the kids and every part of my body hurt," she says. "I was breast-feeding and sleep-deprived because I was up all night with the baby."
And that was just the beginning.
After the training, Thurman flew to Beijing to begin a shoot that lasted more than five months and took her from a mountaintop Chinese Buddhist monastery to a scorpion-infested ghost town in Mexico.
"It was a brutal shoot," says Thurman, who jokes that the movie should be called "Kill Uma."
"I get buried alive. I walk through the desert. I take on 80 men at one time. I get shot in the head. I get shot in the chest. I get shot in the leg. I get stabbed with needles filled with drugs.
"The only thing Quentin didn't do was set me on fire and drown me."