The British actor plays tough cop Jack Regan in The Sweeney, based on the classic 1970s U.K. cop show of the same name.
Winstone filmed several of his own stunts in London last year (11), and he reveals the scenes were so believable, Rihanna's minders leaped into action to protect her from the fake gunfire.
He tells Britain's Daily Mail, "There's some great action sequences in there, including one following a raid on a bank in Trafalgar Square on which we supposedly blew half of the movie's budget!
"It's believable action, so much so that even the security guys who surround pop star Rihanna thought we were for real. We were filming near The O2 Arena, letting off guns, and Rihanna's people thought she was coming under attack! They bundled her out of the building pretty sharpish (quickly)."