The movie star decided to lead by example during the film's most difficult scenes and, before asking actress Giselle Itie to act out torture scenes, he decided to experience what he was putting her through as the director.
And he admits he hopes he's never imprisoned and forced to give up information by waterboarding torture - because it's a terrible experience, even when it's just for a movie.
He explains, "People ask, 'Is it torture?' and I have to say, 'Yeah...'
"Is it horrible? Yeah. As soon as they start pouring water and it gets up your nose, you have no chance. You panic and you start to breath through your mouth - and then you start swallowing water too."
But Stallone was more than impressed with Itie's resilience: "She was great - she did it three times."
U.S. President Barack Obama's government banned the use of waterboarding, which involves the placing of a liquid-soaked cloth over an immobilised victim in order to obtain information from extrajudicial prisoners like terrorists - because the technique was deemed barbaric.
Check out the new promotional trailer for The Expendables; written and directed by Sylvester Stallone. The action-adventure film, full of A-list action stars, follows a team of mercenaries on a mission to overthrow a dictator.
The film is due out on August 20, 2010 and stars Sylvester Stallone, Jason Statham, Jet Li, Mickey Rourke, Terry Crews, Randy Couture, Eric Roberts, Dolph Lundgren, Charisma Carpenter, Giselle Itie, Brittany Murphy, David Zayas and Amin Joseph.
Check out the trailer, below:
UPDATE: Sorry folks, looks like Lionsgate didn't want this trailer released yet and we had to remove the video. We'll get more video for this posted as soon as the studio releases new material.