The controversial romance of an African prince and British office worker helped change history - and now, as the story comes to the big screen, star Rosamund Pike tells Susan Griffin why it was a privilege to be part of it

A picture can tell a thousand words, and one particular image was enough to reduce Rosamund Pike to tears. It was a photograph of Seretse Khama and Ruth Williams, an African prince and the British office worker, whose romance rocked the world when they met in the late-Forties.

"They were sitting side by side, the two of them close up to each other. It was like someone had flicked on a switch," recalls Pike, 37. "I felt tears streaming down my face. Something about them moved me so much."

The actress had been sent a book of images, along with the script for A United Kingdom, from actor and producer David Oyelowo, with whom she'd worked on 2012's Jack Reacher.

"It bore out everything I'd hoped for," she remembers. "I find their story incredibly inspiring and moving, and that's what I ask for in movies I go and see. I want to see movies like that. I want to see movies that make love heroic and the act of love courageous.

"And this script is about love, the extraordinary fortitude and strength it can give you. She and Seretse fell in love in such a true and committed way. She wasn't a political person, out to make a big splash. But in the pressure of fighting to be together, they ended up fighting for so much more."

When Seretse and Ruth met in 1947, the attraction was immediate. She was captivated by his vision for a better world, while he was struck by her willingness to embrace it. Both felt liberated by the social upheaval that followed the war, and while Seretse sensed the opportunity for change as the Empire weakened, Ruth saw the possibility for something 'bigger', as women pushed for independence and equality.

"Ruth was one of the young women during the Second World War who got to take on roles that men traditionally occupied. She drove an ambulance, she could change a tyre and was rescuing stricken fighter pilots coming back from war, so really saw death and life in vivid way," remarks a poised and porcelain-skinned Pike, looking chic with her hair pulled back and a gold pendant, the only jewellery she's wearing, resting on her black top. "I think she had the appetite after seeing all that, to take life, own it and be brave. And I love that about her."

Ruth and Seretse's marriage was challenged not only by their families, but by the British and South African governments. The latter had recently introduced the policy of apartheid and found the notion of a bi-racial couple ruling a neighbouring country intolerable.

As a result, South Africa threatened the British, stating that unless they thwart the couple, they would be denied access to South African uranium and gold, which was vital for the British nuclear programme and to replenish reserves following the war.

But in spite of the pressure they faced, the pair never wavered. Pike found Ruth's sheer strength of conviction fascinating.

"I think girls these days, we don't have that sort of certainty. I think we search for it and strive for it but often don't make it," says the London-born actress, who studied English literature at Oxford University, before being cast as Miranda Frost in 2002 Bond movie, Die Another Day.

"That's the wonderful thing about acting. You get to be all these people who are better than you and have more noble qualities than you, and that's the joy of it," adds the star, whose parents are opera singers (her father, Julian, is a music professor and head of operatic studies at the Birmingham Conservatoire).

It was originally mooted that the film would be made in South Africa, an idea that "sounds insane now", says Pike, who says it was a thrill to film on location in Botswana.

"It was real, and the more real a set can feel, the better it is. You get a texture and production value that you'd never be able to recreate in South Africa, however much you spent."

There were practical difficulties, however. "The infrastructure wasn't there," she notes. "And the extras had no experience. A thousand would turn up one day and say they'd be back the next day, and then only 400 would turn up.

"You never quite knew what was going to happen and that made it exciting. Sometimes you got a spontaneous reaction you hadn't asked for. Like when the women give gifts to Ruth; it wasn't planned, it just happened and that was amazing."

The film's directed by Amma Asante, who helmed 2013's Belle and 2004's A Way of Life.

"She's really wanted to get the female voices in the script resonating loud and clear," remarks Pike. "And she brought her unflinching honesty about her own situation. She'd openly say her family's from Ghana but she was brought up in London and [add], 'There's no way I could go and live in Africa now. I wouldn't do what Ruth did'."

When Ruth and Seretse moved to Botswana, it was Ruth who was an unfamiliar face.

"Usually it's a white world and maybe a black person trying to fit in, and this is the experience of a white woman trying to belong in a black world and craving to be accepted, and that isn't something I've seen," observes Pike, who was nominated for an Oscar last year for her acclaimed performance in the hit thriller Gone Girl.

"I found it really moving and important, and you see Ruth really coming to love that country. Seretse died sadly too young and Ruth stayed on, that was her home. She was not going back to England. I mean England treated them appallingly, and she rejected it, really."

Pike, who has two sons - Solo, four, and Atom, who turns two in December - with her partner Robie Uniacke, also considers it "something special" to be part of a story about interracial marriage.

"That was really key because you don't see that on screen, you just don't, and I want to see it on screen," she states.

"I wish if I was doing a romantic comedy that it was as likely David [Oyelowo] would be selected as the male lead as, oh I don't know, Ryan Gosling. It would be great if there was colour blindness, especially in modern movies.

"There's no reason for the discrimination."

A United Kingdom is released on Friday, November 25