The journey to the big screen for Official Secrets began in 2003, when Katharine Gun became headline news when she leaked a classified e-mail revealing how the US government’s National Security Agency urged UK cooperation in an intelligence “surge” to gather information on UN Security Council members, with a view to securing a UN resolution to send troops to Iraq.

The memo ultimately came into the possession of The Observer journalist Martin Bright. A thorough investigation into the authenticity of the e-mail followed, led by Bright and his colleagues, before the story went to print on March 2, 2003, headlined: “US dirty tricks to win vote on Iraq War.” With her department under intense scrutiny, Gun confessed the leak and lost her job. Moreover, she was arrested and – eventually – charged with a breach of the Official Secrets Act.

“Whistleblowers come in all shapes and sizes, but they do tend to be unusual people,” remarks Bright. “They tend to be loners. They’re often quite strange people. Katharine was amazingly sane. She was very clear about why she’d done what she did.” As he points out, the “principled” Gun only ever leaked one document. “That’s what makes her special. She’s someone who decided to take this stand at considerable cost to her career and her personal life.”

As the story was reported by the media, word on Katharine’s actions spread. Daniel Ellsberg, the whistleblower behind the infamous leak of the Pentagon Papers, called Gun’s actions: “the most important and courageous leak I have ever seen…no one else – including myself – has ever done what Gun did: tell truths at personal risk, before an imminent war, in time, possibly to avert it.”

Unfortunately, Gun’s actions did not prevent the invasion of Iraq or the huge loss of life that followed. After a year of anxiety, she went to trial, defended by the highly experienced lawyer Ben Emmerson. Remarkably, the charges were dropped, with many suspecting that the government did not wish to risk further embarrassment after the search for Weapons of Mass Destruction in Iraq – one of the main reasons to go to war – had proved fruitless.

“It was just a surreal moment,” remembers Gun. “We were prepared to fight. I had declared I wanted to submit a ‘no guilty’ plea. We were prepared to take it all the way. It was almost like pulling the rug from under your feet. I was torn. On the one hand, I was elated because I thought ‘The whole media circus that would’ve ensued if it had gone to a full judge and jury in court, that’s been avoided.’ But on the other hand, I thought the issues we’re going to bring up in court aren’t going to come up now. None of it’s going to be scrutinized.”

For Gun, the process of bringing her story to the big screen began when Marcia and Thomas Mitchell approached her about writing a book on her experiences.

Published in 2008, The Spy Who Tried to Stop a War: Katharine Gun and the Secret Plot to Sanction the Iraq Invasion soon led to various attempts to turn her story into a film. This included Official Secrets, a script by husband-and-wife team Gregory and Sara Bernstein, with producer Elizabeth Fowler (Devil’s Knot) attached.

Such is the slow and often difficult nature of launching an independent film, Gun was skeptical. “I really thought it wasn’t actually going to happen.” But in 2015, producer Ged Doherty was sent the script by the film’s executive producer, Claudia Bluemhuber. “The first time I read the story, I couldn’t believe I didn’t know anything about it,” he says. “I was in London at the time of those anti-war marches, most of my friends were on those marches, and to this day I felt guilty I never went on them.”

Teaming up with Elizabeth Fowler and fellow producer Melissa Shiyu Zuo, Doherty saw Official Secrets as a chance to tell an important story that “shines a light on what actually happened around that time”. Gun, meanwhile, was delighted with his interest and enthusiasm in getting her story to the screen. “Ged came on board and said, ‘We really want to tell your story’,” she says. “I thought, ‘Wow, this is amazing. It’s fate. It’s finally coming to fruition.”

As luck would have it, Bluemhuber had also sent the script to director Gavin Hood, the South African-born filmmaker behind the Oscar-winning Tsotsi.

Bluemhuber had worked with Hood on his previous movie, 2015’s Eye in the Sky, which Doherty had produced through his production company Raindog Films. With political dramas like Rendition also on his record, Hood “was born to do” Official Secrets, says Doherty. “It’s totally in his wheelhouse and the kind of thing that he and I love – a bit of controversy, a bit of politics, shining a light on people who’ve done things they shouldn’t and lie to us all.”

Like Doherty, Hood was hooked by a story that didn’t feel “abstract and theoretical” like, say, the Chilcot report, the Iraq inquiry led by Sir John Chilcott published in July 2016. “When I read the story,” he says, “it felt like this was a way into a very large event through a particular individual. It allowed me to look at this period of time in quite a personal way, through the story of an essentially ordinary person who found themselves in an extraordinary situation.”

Hood immediately recognized the difficulties in turning Katharine’s hellish year-long odyssey into a movie.

“The structural challenge from a writing point-of-view and directing point-of-view was that the third act, which is the build-up to the court case, results in a non-event in a way,” he says. “There is no court case.” Working with the Bernsteins initially, Hood began to refine the script.

Further difficulties included telling a story where, in reality, Katharine only meets Martin Bright just before she goes to trial (Bright didn’t even know who leaked the e-mail until she was arrested). But Hood was determined not to veer off into more fanciful territory. “The real challenge was ‘How can we tell this story in a way that is accurate and true and still have it be dramatic without deviating from the truth in order to create a ‘Hollywood’ movie?’”

Research became paramount, and Hood spent a year investigating his subject.

He interviewed extensively Martin Bright, and fellow Observer journalists Peter Beaumont and Ed Vulliamy, who were also key in cracking the story. “Once Gavin was involved, it was very clear that he wanted it to be as close to the original events, with as little poetic license, as possible,” comments Bright. “He was very keen that all the events involving the journalists, all the events involving me, were represented as they happened.”

This included the horrific moment when The Observer ran the story, only for the leaked e-mail to be deemed fake because it features English not American spellings (the results of an over-zealous intern running the copy through Spell Check). “That Spell Check moment…you couldn’t make that up,” laughs Hood. “It was one of the most awful points of my journalist career, really,” laments Bright. “Comic but appalling.”

As Hood crafted the story to reflect the details his research was revealing to him, he had one overriding ambition with the script. “The important thing, for the sake of the integrity of the film, was to be sure that we the filmmakers could appear with the actors and Katharine and the key investigative journalists, side-by-side, knowing that the story had been told with integrity. Obviously in compressing a series of events that took place over more than a year into a two-hour film you have to take some dramatic license and compress time and select key moments to focus on, but the material facts have to be accurate in a film of this kind.”

Hood also spent time with Gun’s lawyer Ben Emmerson, both in person and via e-mail. “When I met Ben, it was quite intimidating because he speaks fast, he speaks with great intelligence, and he doesn’t suffer fools gladly,” says Hood, whose own legal background – as a student, he studied law in South Africa – came in handy. Others on Hood’s radar included James Welch, lawyer at the human rights organisation Liberty, who came to Katharine’s aid, as well as security expert Paul Beaver.

Above all, crucially, Hood interviewed Katharine Gun.

“When I first met her, it was quite difficult to win her over,” the director admits. “I sat with her for five days. Each day, I’d come in and we’d work for four or five hours, and then we’d call it quits and she’d go and spend time with her daughter. Over the course of those five days, I’d like to think won her trust. I just let her tell me what really happened without trying to bend it into something that might be more exciting from a ‘Hollywood film’ point of view.”

Spending all this time with Hood, Gun soon put aside her natural skepticism. “He’s so full of energy and he’s so passionate and so determined to get to the nitty-gritty of everything,” she reflects. “We spent a week and we went through it all, and he wanted me to tell him everything, from A to Z, so that he could get a really clear picture in his head. I was very reassured when I met him. It showed us he really had that drive to get it going.”

In particular, Hood and Gun discussed the flow of events, which took place over the course of a year but had to be compressed into a two-hour movie. “It’s hard to portray that in a film,” says Gun. In reality, a lot of the experience was “very internal”, as she turned events over in her mind after she was fired from GCHQ. “I immediately lost all my friends. So it was very isolating. But it’s hard to portray someone just sitting there moping! So we had all of these discussions.”

Another key conversation was regarding Gun’s husband Yasar, who was almost deported during the aftermath of the leak but saved at the eleventh hour. For Hood, it was vital to feature this to show the genuine love between Katharine and Yasar, rocked after the leak. “The emotional feeling of that reconciliation is what re-established their love, their connection, after he’d been pretty frustrated and thrown that she’d [leaked the e-mail] without telling him she was going to do it.”

Hood’s attempts to convey not only Gun’s story, but the way it unfolded, impressed Bright.

“The way that it’s written does capture what it’s like to break a big story,” he says. “We had long discussions about the dynamics of a newsroom, because one of the frustrating things as a journalist watching newsroom drama is that it’s very rarely captured well. So we were very keen to make sure the nuts and bolts of that newsroom life were accurately represented.”

Bringing actors to Official Secrets was always going to be a challenge.

“When you’re dealing with real-life characters as smart and unique and with the powerful personality of a Ben Emmerson or the genuineness, intelligence, kindness and determination of a reporter like Martin Bright or someone with the spine, courage and integrity of a person like Katharine Gun, you really need to find actors that embody those qualities in themselves,” says Hood.

The first actor on board was British two-time Oscar-nominee Keira Knightley, cast as Katharine Gun. “We just thought she would be the perfect person,” offers Doherty. “She’d never played anything like this, and if she would be willing to take a risk on it, it would be a phenomenal role for her. That was the main reason and she gravitated towards it straight away, which was brilliant.”

Knightley was 17 when the Iraq war began in 2003 and admits now that Katharine’s story was not one she recalled. “I didn’t remember Katharine Gun. I was pretty politically aware but at that point I was in America, and it obviously wasn’t really covered there at all. So I thought it was really interesting that there was a story that is such a significant part of modern history that wasn’t really known about or remembered. It was an important story to tell and to put out there.”

When it came to research, Knightley diligently read the Marcia and Thomas Mitchell book and waded through the hefty Chilcot report. She also read government e-mails from the time that are now in the public domain “to give me the background I needed to play the scenes; the knowledge I felt she probably would’ve had in some way that made her so absolutely certain that leaking that document was the right thing to do”.

While Knightley also got to spend time with Gun, speaking to her about events wasn’t easy.  “She’s in quite a tricky position because if you question her really on it, she still is bound by the Official Secrets Act. I’m not a journalist, I’m an actress. So I didn’t feel it was my place to push her into revealing to me any more than she felt comfortable with and that she had already revealed to go into the script.”

Meeting Knightley was also strange for Gun. “I met her before filming took place because she wanted to meet me,” she recalls. “We went out for a meal with Gavin and I felt relaxed quite soon after she walked into the restaurant because she came up to me and gave me a big hug! She had lots of questions. She was really keen to know absolutely as much as she could about how I felt at the time and what was going through my mind.”

When it came to casting Martin Bright, Doherty suggested The Crown star Matt Smith to Hood, and the director agreed. By chance Bright knew him already. Back when he was political editor of the New Statesman, Bright had been an advisor on the TV show Party Animals, which featured Smith. “I’d shown him around Parliament, as I had with the other actors, as part of my job on that TV programme,” says Bright. “We were able to have quite an easy connection as a result of that.”

Bright contacted Smith, who was on a beach holiday in Mexico at the time. “It was a strange situation to be in,” says Bright. “I was saying to Matt, ‘This is slightly awkward. Would you like to play me?’” Fortunately, Smith responded swiftly and positively. “I just thought it was a very present and pertinent and interesting story,” the actor says. “Women speaking up and taking a stand…there are a lot of things that feel very present.”

As Hood recalls, “I love the comment that Martin made about Matt. He said, ‘You know, I think Matt is a better Martin Bright than I am!’” Bright clarifies: “On a very practical level he was just about the same age as I was at the time. He’s got a kind of intensity that is important…that kind of journalism involves a degree of intensity. There’s a seriousness that’s involved that he was very aware of, so it did strike me that he was someone who could take on that role.”

Having played real characters before, Smith was all too aware of what was required. “It’s not like Prince Philip or Robert Mapplethorpe or Charles Manson where you can try and bottle a vague essence of someone. With this, it was about bottling my version of this journalist in this story. I tried to tell it as simply as possible. There’s a no-frills nature to him.” Smith even took advice from Bright who texted him some wisdom from the Sunday Times’ Nick Tomlin: “The main attributes of a good journalist are: rat-like cunning, plausible manner and a little literary ability.”

Gun, who now lives with her husband and daughter in Turkey, saw the film on a trip to London.

“When I saw it, a lot of it is very close to the mark,” she reflects. “It brought things back to me and it was quite weird seeing the same sort of scenes repeated in front of my eyes….it was a bit of an emotional rollercoaster ride.”

Watching the finished film with relatives, including her father, it invariably stirred up old feelings. “We were all saying how angry we were again about the whole debacle and how really it’s an open sore. I hope it will make people realise how nothing much has changed. It’s been a continuum of the same for the last fifteen years and it’s a really shocking state of affairs. There’s a whole new generation who won’t have caught onto this yet, because they were under 10 when it happened.”

As Bright puts it, Katharine’s story – and the Iraq war itself – has huge ramifications. “This is a war that corroded all our major institutions – our judicial system, our political system, the intelligence services and the press. So it continues to have a major effect on our public life. If there’s a central core to this story, it’s that. What Katharine was revealing went beyond a simple piece of wrongdoing. What she was revealing was something wrong at the heart of our national and international institutions.”

As difficult as the process was, Gun is delighted that the British government’s lack of transparency will once again come under the spotlight when Official Secrets is released to audiences worldwide.

“It’s this whole paradigm of what is national interest?” she says. “So much gets swept under that overall heading. Who is to decide what is in the national interest?” As Fiennes puts it, like fellow whistleblowers Edward Snowden and Chelsea Manning, Katharine Gun needs to be celebrated. “That courage is really rare.”