The former mixed martial arts fighter plays Mallory Kane, a covert operative forced on the run after being betrayed by her colleagues, in Steven Soderbergh's new thriller.
Recalling a scene in which she goes head-to-head with Fassbender, Carano claimed that she "went black" when the Shame actor got a little too wrapped up in the action.
"I got to be physical with some of the hottest guys in Hollywood," she gushed to People. "When Fassbender slammed my head into the wall, that's the only time I went black.
"And then I slammed a vase right into his face when he wasn't expecting it. He didn't get cut, maybe a little bit."
Carano - who herself is believed to have knocked out a stuntman during the shoot - went on to name the "very athletic" Channing Tatum as the co-star she'd least like to do battle with.
"[He is] like a bro. I'd probably want to choke him out real quick," she said. "Because he's actually very athletic."
Michael Douglas, Ewan McGregor, Antonio Banderas and Michael Angarano also star in Haywire, which is currently playing in the UK and US.
> Haywire Michael Fassbender 'wore padding' fighting Gina Carano
Watch Gina Carano and Channing Tatum fight in the first five minutes of Haywire below: