As it turns out, the film ignores her tumultuous marriage to Prince Charles to focus on the last two years of her life - and specifically her relationship with Pakistani heart surgeon Hasnat Khan. Based on Kate Snell's 2002 book, , the producers said this would not be a sensationalist kiss-and-tell tale, a claim backed up when Oliver Hirschbiegel was hired. The German-born director made the 2004 film about the last days of Adolf Hitler - "an absolute masterpiece of a film", says Naomi Watts, who plays Diana.

The British-born, Australian-raised actress has never been afraid of a challenge, winning two Oscar nominations for her work in and last year's tsunami drama . But taking on one of the most photographed women in the world is another matter. "If you're not scared by it, it's probably not worth doing, is it?" she says, although the attention that surrounded her during the shoot was overwhelming.

The paparazzi, in particular, were out in force. "It's part of the job but it can be very irritating," Watts says. "There were certain nights shooting in Croatia when we just had a wall of them - the sound of shutters going off was very off-putting. But then that was Diana's life."

Photographers were one thing, but then came the newspaper articles. "There are quite a few people out there ready to criticise the film," says costume designer Julian Day.

Having borrowed some of Diana's real-life dresses from the personal collection of Jacques Azagury, the man who designed them, Day was incensed when he saw one article juxtaposing a picture of Diana in a blue dress with a still of Watts from the film dressed in the same outfit. "Somebody wrote 'And it doesn't even look like the same dress; it's so cheap and nasty!' And the ironic thing is, it is her dress! It's the one Diana wore. People are just dying to criticise."

As Day accepts, a film about a woman as beloved as the late Princess of Wales is always going to draw fire. "Everyone is going to want to own a piece of Diana really, aren't they?" he says.

Watts, who wore a prosthetic nose to make her look more like Diana, became obsessed with getting things right. Working extensively on the accent, she paid particular attention to the infamous interview Diana gave to British current affairs show in 1995, watching it every day in the make-up trailer, even playing it on her iPod when she went jogging. "It was probably the thing I was most committed to getting exactly right - the head moves, the eye moves, the hair - because everyone knows it so well," she says.

As committed as those on the production were to getting things just so, it ceased to matter to some. Khan, who for the past 16 years had maintained a dignified silence over his time with Diana, condemned the film. In a recent rare interview, the surgeon told Britain's that he had "never given any approval" to the production. "It's based on gossip and Diana's friends talking about a relationship that they didn't know much about … it is all based on hypotheses and gossip."

The cast feel otherwise. "It's very respectful to the parties concerned," says Naveen Andrews, who portrays Khan and is best known to audiences around the world as the Iraqi-born Sayid in TV sensation . "For one iconic figure and someone less known to the public but an honourable and decent human being, I think it's very respectful to both of them." He calls it an intimate love story. "It reminded me of the David Lean film … in its quietness and its intimacy, and you just don't see that in the cinema anymore." Reading the script, he says, also "gave me compassion for Diana, which I'd never really felt. My whole attitude to the royals was obviously influenced [growing up] by the Sex Pistols, and all that kind of thing, and you didn't really see them as human beings, but an institution. Of course she was outside that institution, but I didn't really realise to what extent. So I had a whole new empathy for her as an ordinary human being who was very lonely and just wanted to have a life, like everybody else."

Still, if the knives were drawn and sharpened before hit screens, when its world premiere took place in London on September 5, the press took delight in plunging the blade right into the heart of the film. "Atrocious and intrusive," claimed ; "An excruciatingly well-intentioned, reverential and sentimental biopic," ran . Watts did not escape censure. "Wesley Snipes in a blonde wig would be more convincing," claimed .

Indeed, it's hard to escape the flaws of the film - which feels more akin to a Mills and Boon airport novel than the delicate -style romance that Andrews claims. Scenes of a domesticated Diana washing up in Khan's flat and even putting on a Liverpool accent on the phone to be put through to his office don't sit well - with the tone veering wildly as the Queen of Hearts is romanced by a man who operates on them.

Speaking at the recent Zurich Film Festival, Hirschbiegel shrugged off the criticism. "The reaction of the UK press is a total déjà vu. It's exactly what happened back then, if we want to remember what the [ ] would write about her back then - really vile things. In a way, I guess I succeeded."

Diana