"Initially, it was written with George Clooney in mind," Bullock, 50, tells Entertainment Weekly. "About two-and-a-half years ago I just put out the feelers saying, 'I'm not reading anything I'm excited about. Are there any male roles out there that they don't mind switching to a female role?' "
And so "Calamity" Jane Bodine was born.
"She's an amalgamation of an essence of a few people," the reigning World's Most Beautiful Woman says of her character, who has been pulled out of retirement to help get an unpopular Bolivian president reelected. "She's someone who's superb in her field."
Bodine is great as what she does, but she didn't get the nickname "Calamity" for no reason.
"She brings a degree of chaos when she steps into the world," says Bullock. "All she cares about is winning."
The actress was excited to play such a complex character.
"You don't get to read roles like this a lot," says Bullock. "There's a lot of unforgivable things that she does, yet somehow you have to cheer for her."
VIDEO: Sandra Isn't Just the Most Beautiful ... She's Also the Most Fun
The mom to 5-year-old Louis stars opposite Billy Bob Thornton, whom she said was "such a joy to work with."
"He's so good at just being bad, but yet you can't help still love him," she says. "People like that don't have to say anything – they just walk in the room and they put out this air that gets everyone on edge. Very few people can carry the weight that he does without saying anything, and that's what just made this so juicy and fun."
Our Brand Is Crisis, which will screen at the upcoming Toronto International Film Festival, opens in U.S. theaters on Oct. 30.
Reporting by CLARK COLLIS