The actress, 29, said she pulls no punches when she has her eye on a movie role.
Talking about new film The Other Boleyn Girl, she said: "I'm actually a scheming bitch!"
However, co-star Scarlett Johansson told her: "I don't think you're that kind of a person. You're just healthily ambitious!"
The pair agreed that mastering the British accent had been their biggest challenge in the drama, in which they play Henry VIII's wife Anne Boleyn (Portman) and her sister Mary (Johansson).
Portman said: "It totally takes me out of a movie when I hear British actors doing a bad American accent. It's a scary thing. I can't improvise with an accent at all!"
The movie, based on the bestselling book by Philippa Gregory, also stars Eric Bana as King Henry VIII.