Scarlett Johansson "loses control" of her face when filming action scenes.
The Avengers actress hates seeing female movie stars still looking perfect when they are getting to grips with something physical and she never worries about being "sexy" on screen.
She said: "I totally lose control of my face. When you see a pack of giant 6ft 7 stuntmen coming after you with huge staffs, you're just like, 'Argh!'.
"There's nothing worse than when you see an action movie and there's a woman who has her lips blown out and her hair in a wind machine.
"I never think about my characters as being sexy. It's not as if I actively look for sexy roles, it's not a requirement that my character be pretty and delicate."
Scarlett - who reprises her Iron Man role of Black Widow/Natasha Romanoff in Avengers - admits she was "permanently bruised" during filming but found it easier from her previous training for the other Marvel blockbuster.
She added to Closer magazine: "Before Iron Man 2, I'd never had any fight training, so it was a whole new world.
"But this time around, I had that experience and could learn faster. That was to my advantage because I had so many fights to learn. It was nuts.
"You just have to get in the mindset that, for the next five or six months, you're going to be permanently bruised."