Gilbert Flores/Celebrity Photo
06/19/2008 AT 07:45 PM EDT
has no problem appearing in the buff for her latest film. In fact, she says it's becoming a habit.
"I always bare my breasts," she said at the press conference to promote The Edge of Love
Wednesday at the Edinburgh International Film Festival. "It's not like it's only in this film!"
The British actress, 23, is best known for the Pirates of the Caribbean
films; last year she went au naturale
in the period drama Atonement.
Referring to a scene in the wartime romance The Edge of Love
with her onscreen husband Cillian Murphy, Knightley said: "It was very simple. It was a sex scene and I never like them when they've got bras on."
So when the director asked her to remove her bra, "I said, 'All right then.'"
But when it came to her singing scenes,
the British actress needed much more persuasion. On hearing she had to sing live in front of the cast and 100 extras, she said, "I thought I was going to die. My knees actually started buckling."
The film was written by Sharman Macdonald, Knightley's mother, who says her actress daughter was the inspiration for writing the period romance that also stars Sienna Miller
, 26, and Matthew Rhys (Brothers and Sisters.
"I wasn't particularly pleased that Shar put singing into the script – in the first couple of takes I sounded like a apubescent boy," says Knightley, who calls her mom Shar and instead of mother.
At the film's premiere in Scotland on Wednesday, Knightley (in a cream Chanel jumpsuit) and Miller (in a Jasmine de Milo dress) walked the red carpet holding hands.
The two fashionable costars "are incredibly warm in the film and that's the way they are in life," says mom Macdonald. "They knew each other slightly before and then they just completely bonded on the film. I hope they will be friends for life!"