At one point in “The Imitation Game” (2014), Benedict Cumberbatch, as the computer pioneer Alan Turing, says to Keira Knightley’s character, “Sometimes it’s the very people who no one imagines anything of who do the things no one can imagine.”
Knightley’s character in that film has a high-security job that’s a bit like that of Katharine Gun, the woman Knightley plays in “Official Secrets,” which opens Sept. 6. The based-on-fact story tells how Gun, a translator at Britain’s super-secret Government Communications Headquarters, revealed information in 2003 about illegal activities by the United States to get UN Security Council approval for the invasion of Iraq. She would later go on trial for her whistle-blowing.
“Secrets” marks something of a departure for the actress. Knightley, 34, has been acting professionally since she was 8. She’s best-known for such period pieces as “Pride & Prejudice” (2005), “Atonement” (2007), “Anna Karenina” (2013), and multiple “Pirates of the Caribbean” movies.
Earlier this summer Knightley spoke by telephone from London about the challenges posed by playing a real-life character and a role different from the sort she’s accustomed to.
Q. What drew you to the film?
A. I just thought it was a fascinating story. I was nearly 18 when we went to war in Iraq. I was politically engaged, yet I didn’t know anything about the [Gun] story at all. Friends who are very politically engaged were unaware of it, too. It’s an important jigsaw piece in the puzzle.
Q. Did you talk to Katharine Gun at all?
A. Yeah, I did. We had lunch before we started, and she came to set with her husband and daughter. I asked her questions — but legally she couldn’t answer them!
Q. Has she seen it?
A. She has. I hear that she’s happy with it.
Q. Is it easier or harder to play a character based on a real person?
A. I don’t know. I think I suddenly felt very self-conscious when she came to set. And she came to set on the day she — I? — was copying the memo, and I suddenly felt very, “Oh god, she’s watching me do something she actually did.” But [director] Gavin [Hood] was very clear he didn’t want a characterization that was just like her. Her hair is different from mine, her accent is very different. Too much specificity would take away from understanding her point of view.
Q. We get a clip of news footage of Gun leaving the courthouse, and she’s wearing the same outfit you are when portraying her in the courtroom. That’s a pretty impressive attention to detail.
A. As far as costumes went, we did do as close to what she had as possible. That was more to give that sense of character. She doesn’t wear makeup. She doesn’t have her hair done. In the movie my hair is virtually unwashed. I wear hardly any makeup.
Q. Is it easier or harder to play a character who’s not at all glamorous? What I mean is, getting all made up and wearing fancy clothes can make it easier to play a part — or maybe playing someone more like yourself can?
A. It honestly depends on what feels right for the character. Anna Karenina was certainly a lot of hair and makeup, but part of the character is her vanity. With Katharine it’s the opposite. But we did play around with more or less makeup from a completely practical point of view. (The decision to go with minimal makeup was a total joy, because I could come in later!) I never want my vanity to get in the way of the character. I always want it to be the character’s vanity or lack thereof.
Q. In a number of crucial scenes you’re acting not with another performer but a screen, computer or television. How different did that feel? Is that harder or easier?
A. The computer screen was fine because that was reading things. Acting with a TV screen was quite tricky, because there was nothing to react to. So Gavin would read what [US Secretary of State] Colin Powell or [British Prime Minister] Tony Blair was saying [during a newscast]. I did do however many “Pirates” films with [special effects] green screens [laughs].
Q. There’s a particular intensity to the scene where you’re questioned by GCHQ internal security. Did it feel that way shooting it?
A. I sort of hung most of the character on the interrogation scenes, the internal security one, then the police interrogation as well. That was partly because you see who she is: her defiance and courage. So much of her isn’t there at first. Those scenes give the feel of the person.
Q. Put in her circumstances, what do you think you’d have done?
A. The absolute honest answer is I have no idea. It depends on the day. Do we want to tell the truth? I think we all do, but would we do it? We’d want to bury our head in the sand. I suspect I’d go for the survivor instinct. That’s why these stories are so interesting: So often people don’t. Our society values truth, but it also values going along, not rocking the boat. So in that sense the hypocrisy within the society is very interesting. Someone who sticks to her morality and gets punished is an example that makes us think.
Q. Which is more important for a movie like this: political urgency or artistic urgency?
A. You’re hoping for both. As an audience member, I just really enjoy films that challenge me, that make me think about the world and society I’m a part of. I’m not necessarily a filmgoer who goes [to movies] to turn my brain off. Not that’s that bad. But I can do other things for that [laughs].
Q. There’s another striking scene, the tracking shot of Katharine walking to the courtroom. Is it easier or harder to stay in character when it’s just you — no one else, no dialogue — simply moving?
A. I like a tracking shot. I love it when you get to do something in as few takes as possible, as few camera angles. It’s a matter of knowing your character and what you’re doing. She’s put her life on the line. As soon as you understand that, it dictates the performance within the shot. Sure, there’s also the technical stuff, about timing with the camera, but I feel that after however many years I’ve got that down.
Q. It’s been a lot of years.
A. It’s definitely been a lot of years.