Even though it was a global news story in the 1940s when Seretse Khama, the crown prince of Bechuanaland (now Botswana), married white Englishwoman Ruth Williams, causing a political crisis both in London where he was studying and back in Africa.

David Oyelowo and Rosamund Pike in A United Kingdom

David Oyelowo plays Khama in the movie, and says he first heard the story about seven years ago when someone gave him a book about the interracial marriage and its fallout. "It was the image of them on the cover that just arrested me," he recalls. "I thought, 'Who are they? Why don't I know who they are? I want to know more.' And that was basically when my obsession with the idea of their story being told as a film began."

He especially likes this story because it's not told from an outsider's perspective, like many movies set in Africa. But this film, and also Queen of Katwe, which Oyelowo made just before it, tells the story from an insider's point of view. "This is one of the reasons I was very keen on Amma Asante to direct it," he adds. "Even though like me she was born in the UK, she's of African descent, and for a film that traverses both Africa and the UK, I just felt you wanted someone who had more of a balanced experience with both places. So it's not what we've normally seen."

He was also the one who reached out to Rosamund Pike to play Ruth in the movie. "Obviously, Rosamund and I had done Jack Reacher together," Oyelowo says. "So I approached her about this. I had always been a fan of Rosamund's, and enjoyed working with her, even though it was more peripherally in Jack Reacher."

Oyelowo also admits that it was amazing to interact with the descendants of his character. "Ruth and Seretse's son is the current president of Botswana," he says, "and he came to visit the set unannounced. We just heard this helicopter, it landed on our set and he stepped out, having flown himself! He came and sat down next to me behind the monitor and watched Rosamund doing a scene. And he turned to me and said, 'I never thought I would see my parents again.' To get that kind of seal of approval from their son was really amazing. Last year was the 50th anniversary of Botswana's independence, so the film was a big part of the celebrations there."

Watch the trailer for A United Kingdom: