Naomi Watts on her new film Diana

There is talk of earlier interviews having been difficult (which explains Hirschbiegel’s presence) but when she stands up to shake my hand, Watts looks less like someone who’d strop out of an interview and more like a person who might just bolt at any moment. Her handshake is tentative, as is her smile. I’m not sure what she reckons I’ve got planned for her, but evidently she doesn’t expect it to be good.

If I was about to offer her my unexpurgated opinion of her new film, Diana, the biopic of the late Diana, Princess of Wales, helmed by Hirschbiegel (director of the acclaimed film Downfall, focused on Hitler’s final days) which sinks under the weight of leaden dialogue despite Watts’ significant charm and talent, she might be right to be braced. But that’s not my job. My task is altogether different: I want to get a sense of who Watts is, what makes her tick. I’m not sure it’s going to be easy.

Watts’ nervousness is unexpected, partly because reviews of the film aren’t in yet (the news isn’t good, although Watts is spared the worst of the criticism) but also because at 44, she is an actor widely regarded as being at the top of her game. She has all the polish of a movie star – she’s beautiful in a delicate way. Her cream top is almost sculptural, the fabric firm, flaring out from the shoulders above wide-legged black trousers and elegant stilettos. A selection of fine gold chains hang around her neck, a large-faced rose gold watch is pushed up one slender wrist. But she doesn’t have the presence that you might expect. She is small – petite, fine-boned, fine-featured, almost fragile. And what is truly striking is how thoroughly uncomfortable she looks.

Watts’ discomfort with the hoopla of a press junket makes me think of the way in which in recent projects she’s been able to disappear into her characters. Might it be that Watts’ ability to be ordinary on screen – since it’s that more than anything else that has allowed her to create magnificent performances as real women in recent films – is more than just her acting skill?

Last year it was as María Belón, the mother who along with her family was caught up in the Indian Ocean tsunami of 2004, a couple of years before that, in a film called Fair Game, it was as Valerie Plame, a CIA operative whose identity was leaked by the White House. What Watts brought to both of these roles was an utterly compelling yet unassuming heroism that allowed her to be the emotional anchor of both films but also, simply real. Watts’ true talent is that she is an A-lister who can inhabit ordinary women, albeit those caught up in extraordinary stories.

And now it’s Diana.

Of course, a woman who married the future heir to the throne, had two sons, became the most famous woman in the world, rocked the monarchy with her “people’s princess” emotionalism, divorced and then died in tragic circumstances at the age of just 36 could hardly be described as ordinary, but that’s precisely the aspect of Diana with which Watts says she “fell in love”.

“She did try to be ordinary,” she says. “She was a member of the Royal Family and yet she still wanted to go out on her jogs and do normal things. She helped us feel that the Royal Family were human beings, not these untouchable people. She modernised them and made them feel more accessible.”

In representing the final two years of Diana’s life, a period during which Diana was involved in a relationship with cardiac surgeon Hasnat Khan before her one with Dodi Fayed, who died with her in Paris, Hirschbiegel’s film is more than a little hamstrung since neither the Palace, nor Khan, collaborated with the film-makers making it feel like a work of fiction, without the latitude it would have if it really was entirely made up.

But these limitations are nothing to do with Watts. She delivers the performance you’d expect: sincere and understated. It is her attempt to find and portray the real woman behind the public persona. She’s also made no secret of the fact that it was the hardest role that she’s played. That strikes me as quite an admission from a woman who, last year, was spinning about in the debris-filled water of a re-enactment of a natural disaster that killed hundreds of thousands of people.

“That was pretty hard,” she says. “It was up there. But it was hard for a different set of reasons. No-one was familiar with María so I could do my own walk and talk. The responsibility and the pressure that comes with that responsibility was there; she survived the tsunami but hundreds of thousands of others didn’t so you felt that you needed to tell it as accurately and truthfully as possible. And in this case everyone knows Diana, or feels that they do, so to take possession of her was scary.”

She stops for a moment, and looks at me. It feels like she’s looking for something, but I don’t know what it is. “People are like ‘I know Diana, she belongs to me’, so you’ve got all that energy in the background. ‘She’s not tall enough’, ‘she doesn’t speak right’, ‘she’s not British’, ‘she doesn’t have the nose’. All of those things.”

It’s true that Watts doesn’t have a prominent nose, as Diana did. It’s also true that at only 5ft 5in to Diana’s 5ft 10in, she’s much shorter. But Watts is British. At least she was born in Britain, in Kent, and lived here with her mother and brother (her parents separated when she was four) until she was 14 when her Welsh-born mother, Myfanwy, moved her family to Australia. It was there that Watts decided that she wanted to be an actor. She met Nicole Kidman at school in Sydney and soon after moved to Los Angeles.

Whereas Kidman married Tom Cruise and made becoming a movie star look simple, Watts’ experience was much trickier. For years, she struggled to be cast. There were small roles in TV shows and daytime movies, but they didn’t stack up to much and they weren’t all that easy to come by. Watts was 31 when she finally got her break. David Lynch cast her in Mulholland Drive and with that one role, everything changed. Originally shot as a pilot for a TV series, it was only after it was turned down by the networks that Lynch reworked it into a feature film and Watts’ career properly took off. Since then, she has worked with David Cronenberg, Woody Allen, Peter Jackson and Clint Eastwood. She was nominated for an Oscar for 21 Grams and was short-listed again last year for The Impossible. For someone who once described herself as “unhireable”, things have worked out pretty well. What is perhaps even more impressive is that Watts has also managed that feat of balancing a happy family life with a high-flying Hollywood career. Watts lives with her partner, the actor Liev Schreiber, whom she met on the 2006 film The Painted Veil, and their two sons, Sasha, five, and Sammy, four. The family live what seems to be an almost ordinary life in New York. It makes me wonder what Watts’ take is on the press intrusion that played such a feature in the life of Diana, but has surely only become more intense for people with a public profile such as Watts in the intervening years?

“It is worse,” she says, “because there’s much more volume and power behind the media, but Diana was the only one and now there are so many people. It’s like you might have a bad week of an explosive, scandalous story, but then it’ll be gone because someone else will have come up.”

Watts and Schreiber are most often photographed looking studiedly impassive while taking their kids to the park. Presumably it’s the best way to ensure that the men with long lenses intrude on their family life as little as possible. But, equally, surely fame has some positives?

“Of course,” she says, smiling. “When you want to get a table, that’s the best, the absolute best. Or when there’s a little bit of a line and someone sees you. But I never get recognised. I think I just look very different on film than I look in real life.”

I can well imagine that since Watts’ diminutive stature must allow her to be pretty inconspicuous. She doesn’t mention this lack of visibility with regret, more a sort of resignation. But surely being a movie star and being pretty anonymous is the perfect combination?

“Yeah, well I get to do whatever anybody wants to do,” she says. “I don’t get hassled.”

In scene after scene of Hirschbiegel’s film, Diana is shown walking through one opulent hotel suite or palace after another. She looks lost, lonely, trapped and isolated. It would take a heart of stone not to feel sad for someone stuck in that situation, but I confess I also felt irritated that someone with so many resources at her disposal spent endless nights lying on her sofa watching EastEnders. (I confess to Watts that I’m not exactly a royalist. “I’m Scottish,” I offer apologetically by way of an explanation of my anti-monarchist tendencies. Watts laughs heartily.) “But that’s the dichotomy that she lived with,” Watts says. “And it was a constant conflict. She was both those things and yet she did get up and do things despite the construct [the Royal Family] and the experience of being trapped.”

The decision to play Diana was not straightforward for Watts. In fact, she turned it down twice. She knew the level of scrutiny that she’d face, but she also felt a particular sensitivity for Diana’s children, Princes William and Harry. “Going into this for me that was playing a part,” she says. “I was thinking about how they will feel. But it’s history and she was the famous woman of our times and we are going to be fascinated with her life, even 16 years after her death. I’m sure they’re aware of that.”

She also felt committed to the part for her own reasons. “What’s the point if you’re not taking risks? There has to be some risk-taking – the reasons that I wanted to say no were sort of the reasons I ended up saying yes. They were pretty much all fear-based and giving into fear is such a bore.” She smiles. “It’s better to trump it.”

Having weathered that long, dry spell at the beginning of her career, I wonder whether Watts is one of those people whose job is her vocation, or whether she can imagine doing anything else with her life other than acting?

“I wish I could do something else,” she says instantly, with a wry laugh. “I tried to think of other things, but they just weren’t…” she pauses. “I knew I wanted to travel. I knew I loved children. I didn’t really have many other skills. I was going to have to find a rich husband fast. No, I didn’t have the skills. I thought I might be a yoga teacher for a while, but I wasn’t that good at yoga.” She shrugs.

So now, from the other end, there must be a pleasure in knowing that it has worked out, that she made it, there’s no need to come up with a Plan B. “I do enjoy my job, I do love it,” she says not quite convincingly. “There are other great things in my life particularly now with children, but I kept getting hooked back in just at the point where I was ready to give up. Something would keep me there.

“I was sort of OK with being an actor for hire, that’s how things were going before Mulholland Drive. It was a couple of weeks on a TV show or a movie of the week, something like that. I still loved that.” She might even just sound a little wistful. But with that, time’s up. We shake hands and although there’s more warmth this time, I’m pretty sure I can sense the relief that she’s about to see the back of me.

Diana is on general release.