Keira Knightley on cracking the code of a WWII heroine

Keira Knightley's 2003 "Love Actually" quickly became a holiday classic, but Thanksgiving weekend, you'll see her in a very different role -- a World War II code breaker in "The Imitation Game," reports "CBS This Morning" co-host Norah O'Donnell.

"To tell you the truth, I'd sort of said yes to this project without even knowing what the role was because I wanted to be a part of this kind of trying to get the name of Alan Turing out there," Knightley said. "When I read the role I was so pleased."

Knightley's latest movie tells the real-life story of British mathematician Alan Turing, played by Benedict Cumberbatch. During World War II, Turing's team of code breakers cracked the uncrackable -- Germany's Enigma machine. This feat shortened the war by an estimated two years, saving thousands of lives.

Knightley plays Joan Clarke, a fellow mathematician and cryptanalyst recruited by Turing for the covert operation.

"I don't know what that means," she admitted, "I'd love to say that I did."

And despite her role as a code-cracker, figuring out how to portray Clarke was no easy game.

"I did a lot of research into all of this," she said. "I tried to read about the math. I tried to understand the machine. I tried to understand all of it and I went, 'I'm going to have to act this role.'"

Although her character in the film is a genius, through most of it, it's almost as if she has to play dumb.

For Clark, breaking military code proved easier than breaking into an all-boys club. In order to join Turing, she had to pretend she was a secretary.

"And I think that was very true, you know, I think that was as much as she was trying to break that glass ceiling, she realized that by being a bull in a china shop, it wasn't gonna get her what she wanted," Knightley said. "But by being somebody who people liked to be with, she managed to slowly get to the places that she wanted to be."

Knightley said that sentiment is still true today.

"I mean you read newspaper articles about women and they're described as being pushy or they're described as being divas, or they're described as being bossy... and you think that's not words that would ever be used to describe a man," she said. "And they certainly wouldn't be used negatively to describe a man."

To this point, Knightley had a run-in with a Google executive when they screened the film in Silicon Valley.

"Yeah, we went, 'How many women have you got?' and he said, "I don't know, 20 percent to 30 percent." and we were, like, 'That's not okay,' and he's, like, 'No, we are working on it.' I said, 'Are you working hard on it?' He says, 'Yeah, yeah, we're working on it,'" Knightley recounted.

For her, it was important to hear that.

"They always say, you know, 'They're very macho fields.' And you think, 'Yes, if there were only 20 percent or 30 percent women, of course they're very macho fields. You have 80 percent men," she said.

Chiding an audience of CEOs is just one perk of being amongst the highest-paid actresses in Hollywood, where Knightley reigns supreme, corset in tow.

Usually the star, she said her role in "The Imitation Game" is intentionally not a lead.

"I think the thing about all of these characters is this is very much a film about Alan Turing," she said. "You know, everybody else, all of the characters are supporting."

She said while there may have been more to the story, they had to be selective as to not overload the film.

"The most difficult thing was sort of, once you did start doing the research into the people, going, 'Oh, can't we, can't we put that in? And can't we put that in?'" she said. "And of course, in two and a half hours in one movie, of course you can't. You have to be very selective. And given who it was about, it had to completely focus on Alan's story."

While the film has still not been released, critics have said it's a frontrunner for the Oscars.

"Who knows? I think when you make films you just try and hope that the audiences will enjoy them," she said. "So if that can continue, then how wonderful, and if it gets awards, how wonderful. And if not, I think we're still incredibly proud of it."