Because the girl who took on the part of J K Rowling's main female character in the hugely successful films of the Harry Potter books has, in eight years (she was nine when she first auditioned for Harry Potter and the Philosopher's Stone), made the role so much her own that it is difficult to tell where Emma stops and Hermione begins (or vice versa).
Not that she minds. In fact she regards it as something of a compliment.
"J K Rowling based the character on herself, so obviously she will have had a very strong idea of how she would develop, but maybe I have played a small part in the way she is growing up. It would be very flattering if I had.
"J K Rowling has been really lovely and very supportive of the films - she comes on set and she is in e-mail contact with a lot of us. She just said to me, 'You are Hermione, you have completely become her,' which was just so nice and so generous of her."
We meet after she and her family have attended a private screening of the fifth and latest film - Harry Potter and the Order of the Phoenix. It is, Emma thinks, the best film so far - more "psychological" and darker as arch-villain Lord Voldemort attempts to get into Harry's head. Even Emma, who knew exactly what was going to happen, "bawled" her eyes out at the end.
"People always expect me to know what the finished product will be like, but I don't have a clue because it is all shot out of sequence. Before I see every film my nerves are just terrible.
"I remember after the first ten minutes of the first film my dad turning to me and saying, 'Emma darling, I really think you should breathe now'."
What impresses you most about Emma is how unspoilt she is; articulate, opinionated and self-deprecating, she is emerging as a delightful young adult.
Being a part of such an internationally successful phenomenon (three of the films are among the top ten worldwide biggest grossing productions of all time) could have turned her into an arrogant brat or a spoilt It-girl (she is reported to be earning £2 million for each of the final two films).
But Emma and the other two central characters - Daniel Radcliffe's Harry and Rupert Grint's Ron Weasley - have managed to maintain a kind of normality despite their extraordinary celluloid upbringing. The film's producers created a familial atmosphere on set by ensuring continuity of the crew and support staff around the children (Emma's driver, Nigel - her "best friend" - has been collecting her since her first audition).
But there is nothing remotely normal about growing up on a film set of the mammoth proportions of the Harry Potter ones at Leavesden Studios in Hertfordshire. The scale of the studios (the old Rolls-Royce factory) is such that the cast are ferried between stages on golf buggies. Emma points out that as she was so young when she entered that world she somehow just accepted the strangeness.
"You will be in the canteen and there will be all these witches and wizards and ghosts and ghouls queuing up. I get a reality check whenever my family or friends come and a centaur goes galloping by. They will be sitting there staring but I just don't see it because I have never known anything different."
Looking back, Emma now thinks that her parents - both lawyers - had absolutely no idea just how her casting as Hermione would change all their lives. While Rupert applied to audition for his role and Daniel - who had starred in a BBC production of David Copperfield - was well-known in the film industry, Emma was randomly spotted when casting agents came to her school.
Born in France where her parents were working (she returned to England aged five when they divorced), she now lives with her mother in Oxford and spends every other weekend with her father in London.
Both parents have new partners and in her extended family set-up Emma - who is particularly devoted to her full-brother, Alex, 14 ("the funniest person in the world" - finds herself the eldest of seven children.
"There is Alex and then my mother's partner has two sons younger than me who regularly stay with us, and my father and his new wife have two-year-old identical twin girls and a three-year-old son.
"We all get on really well. My family has exploded in the last two to three years so it's nice ? by contrast ? to be the baby when I am working. I am the youngest (she is nearly a year younger than Daniel and 18 months younger than Rupert) and I am a girl, so Dan and Rupert are really protective of me, they are like my brothers. Although they do tease me a lot," she says with an affectionate smile.
In many ways Emma remains an almost typical teenager. Her iPod, she says with a grin, is "my life", and her music taste ranges from her parents' favourites, Lloyd Cole and the Commotions, to hip-hop artists she loves to dance to, such as Missy Elliott, Brandy and Ciara.
Her mobile phone, she reports with another grin, is the means by which she clings on to the "life" that she has to put on hold when she is away working on the film (each movie takes around 11 months to shoot).
Wonderfully polite, she apologises profusely when - several times during our interview - the electric ping of her phone indicates another incoming text.
"I am a mad texter - I can do it with my eyes closed - because I am away so much; it's the way I keep in touch with friends. My one big extravagance is my phone bill. I am always texting Dan and Rupert when we are not working. And we are always swapping girl and boy advice when we're together.
"It's quite funny - I am like, 'Oh, I have got this text from a boy, what on earth do I say? What does this translate to in boy language? I don't understand,' and they will be like, 'Well, this means this.' I have got my own personal on-set boy text translators," she says with a laugh.
Emma is sensibly circumspect about discussing any real-life romance she might be involved in ("I wouldn't tell you if I did have a boyfriend as it would be unfair to the other person," she says, although she was recently escorted to a ball by 21-year-old actor Henry Lloyd-Hughes who had a bit part in the fourth Harry Potter and who plays in the band Adventure Playground with Peaches Geldof's boyfriend).
But she is happy to talk about the slow-burning on-screen chemistry between Hermione and Ron - which falls short of the much anticipated first kiss which Harry shares with fellow pupil Cho Chang in the new film.
"The romance between us is there if you watch carefully, it's all underneath the surface. There are hints of it all the time and at one point Hermione and Ron have a duelling match and she absolutely smashes him into the back wall with this spell. And Ron is pretending, 'Oh yeah, I let her do that, it's manners isn't it?' which is quite funny."
Emma is circumspect, too, about certain aspects of youth culture.
The night before our interview my own teenage children - all fascinated by Emma and the other stars of Harry Potter - had gleefully found an ' Emma Watson' page on Facebook (the latest in a series of youth-focused social networking websites).
"Oh no, that is a fake," she says. "I have got fakes on Facebook, Bebo, MySpace - the lot - and it's quite annoying. I can't understand why people put themselves on there. Some of the pictures that they put up of themselves make them vulnerable because anyone can see. I think the internet is very scary."
It is easy to understand why Emma might be wary of courting publicity - last year it was reported that she was given a bodyguard after a male fan managed to enter her school and approach her. Generally, though, she is pretty relaxed about the way in which her fame sometimes infringes on her private life.
"You wouldn't believe how strongly I fight against not being normal. I take the bus, I take the train and when I go to visit my dad in London I am on the Oxford Tube (coach). Well, either I am going to live a completely shrouded life in which I hide from everyone and am driven around in cars or I attempt to live a normal life.
"Yes, I do get stopped and sometimes it may be difficult to deal with, but I would much prefer to pay that price than not have any freedom. It's normally just tourists who shout 'Hermione!' and chase after me. I have been in town with friends and been chased down the street and have had to hide in shops.
"Dixons is my favourite hiding place. I shouldn't be telling you this because it won't be my hiding place any more, but I go and hide behind the computers because that is the last place they expect you to be."
The other way in which Emma holds on to a normal life is by continuing with her studies.
While the other two stars have put their education on hold (Rupert gave up after his GCSEs and Daniel after his AS-levels), Emma is determined to continue (she gained eight A*s and two As at GCSE and has just finished AS-levels in English literature, art, history of art and geography). When she is working she has a tutor and when she is not she is just a regular pupil at her all-girls day school.
She imagines she will go to university (both her parents went to Oxford; she favours philosophy at Cambridge) but she has no other ambition but to be a proper grown-up actress. Emma laughs out loud when asked if she has any plans to take off her clothes in a London theatre in the manner of Daniel Radcliffe in Equus (when we met she still hadn't seen him in the play).
"The kid has done good," she says.
"It was really brave and I thought he was hilarious in Extras. I would love to do some theatre but it is hard to fit in with Harry Potter and my studies. I am waiting for the right time and I am thinking about next summer after my A-levels. A period drama maybe, and it doesn't have to be a main role, it just has to be something I believe in."
It has, Emma says, been a huge privilege - and a great learning process - to work with the many distinguished actors who have taken roles in the Harry Potter films. She enjoyed working with Helena Bonham Carter, who was "so much fun", and she has great affection and professional admiration for Emma Thompson and Maggie Smith ("who always gives me chocolates at Christmas"). The actors, she says, "are a bit of a family for us as well". Particularly Robbie Coltrane (who plays Hagrid, the gamekeeper at Hogwarts School of Witchcraft and Wizardry).
"He is always telling us completely dirty and inappropriate jokes that we are far too young to be hearing, but we love, of course."
Outside her work and her studies Emma likes shopping with friends ("but I am not a fashion victim"), eating out and playing sport. When asked, in jest, if she has a glass of wine with her favourite food (she loves eating at Carluccio's) she points out that having spent the first five years of her life in France she had her first watered-down wine when she was about three. When I suggest that such an idea won't go down well in America - where the alcohol age limit is 21 - she is not afraid to offer a forthright opinion.
"What is that about? You can go to war for your country but you can't have a beer to celebrate? It's mad. I much prefer the European way in which alcohol is naturally introduced into family life."
Nor is Emma afraid to offer opinions on other contentious issues of the day - such as feminism and size zero. Naturally slim, she is aware that in commenting on weight she might be criticised by schoolmates who have more reason to worry.
"I am lucky that I stay the same weight. There are so many girls at my school who suffer from eating disorders. There is so much pressure on girls our age to be smart and pretty and funny and skinny - they have to be everything. I definitely know what that pressure is like but my philosophy is to eat what you like and be healthy and take exercise."
One thing that annoys her about her female contemporaries is their reluctance - from vanity, she thinks - to continue with sport in their late teens.
"I am such a feminist on this. It drives me nuts when friends say, 'We can't continue because sport gives you muscles and it's so unattractive, and you get sweaty.' For some reason girls seem to think it is unfeminine and they worry about being 'pretty'. But I feel the most pretty when I come off the pitch after a hockey game and I have got pink cheeks and bright eyes. Sport really makes me feel good about myself."
Emma gives a good-natured groan when I mention the fact that she came 98th in the FHM 100 Sexiest Women list for 2007.
"That is the weirdest thing ever and they put some kind of crude comment like, 'She is one of those sporty types - you would have to hold her gym shorts for her,' or something. I was like, 'Good God!' But I suppose it is a compliment for me and Hermione."
Emma talks with such conviction about Hermione that - I gently suggest - it will probably be hard for her when the films come to an end (book seven, the final instalment, is scheduled to be shot in 2010).
"Oh my God, when it comes to an end I don't know how I will feel," she says pausing, before adding - with a big smile - "but to be honest I don't think it will end. I think Harry Potter will go on for ever."
