In contrast, Hirschbiegel had no knowledge or interest in the mythology surrounding Princess Diana's life and death, having grown up in Germany, far removed from the media scrum surrounding her in Britain. That all changed though, when he read the script for Diana, based on the book by Kate Snell.
"I didn't even want to read the script at first, because I didn't think it'd be that interesting," Hirschbiegel confesses, sheepishly. "I just thought she was this Princess who wore nice dresses and was photographed a lot. But when I realised it was written by Stephen Jeffreys, a very accomplished writer, I had a look and then my view of her changed: after 10 pages I was totally sucked in.
"Everybody remembers her in the Mediterranean with Dodi Fayed, this rich playboy, and the paparazzi swarming all around them. But at the same time, in the background, she was continuing her campaign against landmines in Bosnia, which has largely been forgotten, and this true, big love was going on with Hasnat that nobody really knows about. I felt people needed to know the truth, so that's why I decided to do this film."
Consequently, Hirschbiegel spent months trawling through archives, watched hundreds of hours of footage and met with friends and acquaintances, to try and ensure he made a film that truly depicted her life and love affair with Khan. Ironically, despite his efforts there's been a huge backlash about the film's inaccuracies, assertions and storylines.
"But what's here, in the film, actually happened. I did a lot of research and it's based on facts," Hirschbiegel counters. "Even the most bizarre episodes, like her wearing wigs, going to Ronnie Scott's jazz club in disguise and hiding in a car boot to evade the paparazzi en route to secret liaisons with Khan, or visiting him at the hospital, at night, to wait for him are all true. It really happened; we didn't invent them."
However, Hirschbiegel concedes that, at times, it was a tricky act balancing fact with fiction, especially when it came to portraying what actually went on behind closed doors between Diana and Khan.
"As their relationship is something nobody else knows about - and Hasnat still doesn't talk of it - you need to use artistic licence. But you still make sure the scene is honest, truthful and doesn't become a cliche, nor speculate or cash in on the cheap sensation," he adds. "It's got to feel right; it has to make sense within the context of the story and the characters we are depicting, which I think it does.
"I admit I did take one liberty," Hirschbiegel confesses. "I created the scene where they're at the cliffs, by the sea, because I needed to depict their relationship according to what I found out about it. Apparently they did go away together and loved to fool around: she played pranks on him and he made fun of her. So I needed a scene, like that, to convey that aspect of their relationship. But all the other scenes you see, except for that one and the bedroom scenes are based on fact."
The reality is that no matter what picture Hirschbiegel painted with Diana, there would undoubtedly be critics and detractors, simply because Princess Di was so well-known and well-loved. Her informal title, the "Queen of Hearts", suggests as much. Unsurprisingly, for that reason, Watts initially had reservations about the role.
"Taking on the role of the most famous woman of our time is definitely the most challenging thing I've ever done. It was very scary taking possession of that character, especially when everyone feels like they know her so well and therefore she belongs to them."
She's also wary of what Diana's sons, Prince William and Prince Harry, will feel about a chapter of their mother's life suddenly being exposed.
"I don't know whether they'll watch it. I can only hope that they feel that we've treated her story and legacy with respect and sensitivity. Hopefully, if they see the film, that's how they'll feel, but we'll probably never know ..."
Similarly, we'll never know the answer to what really happened that fateful day in Paris, when Diana and Dodi perished while being pursued by the paparazzi. We'll also never know what future Diana might have had if she had answered her phone that evening, when Khan called her to possibly reconcile with her, and whether their relationship might have turned out to be a fairy tale, rather than a tragedy.
It's a possibility that Hirschbiegel portrays in the closing sequence, with a unanswered telephone call - which actually happened - from Khan. It does leave you wondering, what if ...
What: Diana, in cinemas October 10
Who: Actress Naomi Watts and director Oliver Hirschbiegel
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- TimeOut
