Leo, who has appeared in Frozen River and Red State, as well as HBO's Treme and Mildred Pierce, won a 'Best Supporting Actress' Oscar in 2011 for The Fighter.
In The Fighter, Leo played the older Alice Ward, who is the mother and manager of two boxing sons played by Christian Bale and Mark Wahlberg.
"I play who the character is, is what I do. I don't see her and go, 'Oh, you want me looking ratty'," Leo explained to New York Magazine.
"In Frozen River everybody said, 'Oh, you have no vanity'. Well, maybe I don't. I think my mother made sure of it. She didn't teach me to shave my legs, but she did insist I walk without vanity."
When asked if vanity distracts actors, Leo responded: "I don't know that it's the thespians' own sense of vanity, but rather the idea of it.
"Ever since Marilyn Monroe was transformed from one of the prettiest girls you could ever hope to see into an icon, everyone has been trying to repeat that icon. And now the entire industry is filled with, and by and large run by, wannabes. And bless their hearts; I'm glad they have jobs."
Watch the trailer for The Fighter below: