They costar in the new film Cadillac Records, but Gabrielle Union and Beyoncé go way back.

"I've known her since she was a teenager!" Union told PEOPLE at Ideal Bite's "Best in Green" Awards on Thursday. "It's been nice to see that her music and her lifestyle choices and her acting choices have all evolved as she's gotten older."

Most notably? The development of Beyoncé's stage persona, Sasha Fierce.

"Beyoncé is quiet and reserved, very Southern, sweet and polite," Union said. "If someone told me that girl was gonna go on stage and do the kind of performances that she does, and be so fiery, and this quintessential and iconic entertainer, I'd be [in disbelief], like, 'Yeah, okay!' "

There's no doubt that Beyoncé has grown up over the years, but one thing that hasn't changed her, Union says, is marriage. (The singer wed rapper Jay-Z in a super-secret ceremony earlier this year.)

"They were good to each other before, they're good to each other now," Union noted. "They seem very happy when they're around. As long as there's more smiles than frowns, I suppose that's the key."

Beyoncé Is No Sasha Fierce, Says Pal