Interview: Helen McCrory, actress

When I see Helen McCrory's name attached to a project, a flash of happy anticipation illuminates my pleasure centre: I'm confident that whatever else, her performance will be worth the price of admission.

Phenomenal acting aside, she's also a member of that special sisterhood whose magnetism crosses the gender divide, with a beauty all the more stirring for being tough to quantify. If pressed, I'd trace her charisma to the twinkle in her eyes and the wry smile tugging at her lips, hinting at secret knowledge – probably of the saucy variety.

Off screen she does not disappoint. While there's none of that faux best friend nonsense you get with some interviewees, she's companionable and relaxed, tucking into a second cup of coffee with gusto – she's only recently back on caffeine after weaning her youngest – and quick to laugh and tease.

This month we'll see her in a far more high-strung persona, playing Draco's mum, Narcissa Malfoy, in Harry Potter and the Half-Blood Prince. She approached the assignment with relish, keen to work with director David Yates. "Did you ever see (his] Sex Traffic? It was an absolutely fantastic film about two Polish girls forced into the sex industry in Britain." Pregnancy prohibited her tackling the role of Bellatrix LeStrange, as was originally mooted some years ago and it went to Helena Bonham Carter, instead. Now, however, McCrory is able to play her sister.

She appears in the two final films of the series, but warns: "Don't reach down for your popcorn, because by the time you do, I could be off the screen, in Half-Blood Prince I am literally like, whoa, there she goes! But everyone was like that. In our first scene, Helena Bonham Carter and I are rolling past this blue screen, and we knock on the door and Tim Spall comes out.

"Tim pulls open the door and looks. That took about a day and a half of filming, and then it was, 'Thank you very much ladies and gentlemen, that's a wrap on Mr Spall,' and you go, that's it? You have people like Tim, that most producers are killing themselves to work with, and that's the fun of doing (Harry Potter]. Everyone is giving their all and they're all used to playing the lead parts. They're going, 'How shall I open the door? Do you want me to do it like this, or like this?' That sort of attention to detail, because, you know, they probably last did Hamlet. So David is surrounded by actors going, 'Right, I can give you this, or I can give you that, or I can give you . . .' and he's like, 'Please, please, just say your one line and f*** off.'" She laughs heartily. "He's extraordinary, because it's a huge project: the pressure!"

As the mother of two young children – with her husband, actor Damian Lewis, she has a daughter Manon, two, and a son, Gulliver, one – I wonder if she approached Narcissa in the spirit of Cruella de Ville, or that of a ferocious mum defending her cub?

"In this film it's about protecting her son. The fact that you have children probably makes a bit of a difference, but not enormously, because you do spend a lot of time as an actor trying to imagine what the other person's feeling, anyway. As a woman you've played mothers before, and you don't have to be a mother to understand what feeling protective is all about."

By her own description, McCrory, 40, is a very un-neurotic parent, saying she inherited that tendency from her own folks. Her mum is Welsh-speaking Welsh, and her father was born in Glasgow. Her granny and other relations still live here.

"My own parents were very un-neurotic, so I never thought that I had to change enormously in order to become a parent. Also, my children aren't very neurotic, and you don't choose the personalities of the kids you have." Surely parents influence them, though? She's unconvinced. "Yeah, but I think they definitely have personalities coming through pretty early on and both my children are very happy creatures.

"Actually, and I think this is probably true of all children their age, you think you're going to take them to do this or that, and you only get as far as the front step because they'll see a ladybird or want to look at a leaf and then they potter around the corner and see a spider. Children have their own elongated time."

I'm nostalgic for those days, I admit. She chuckles. "If only you could get someone to cook for you, dress you in the morning, and wash you at night and put you to bed, then you, too, could have this relaxed lifestyle!

"Childhood has definitely been invented, hasn't it? I think that's because people had children later and we appreciate and cherish childhood a lot more. And also, people have a lot more time, since I don't have to hand wash our clothes, I don't have to do all the things that our parents had to do – well, not our parents but definitely our grandmothers."

Does she do that herself or are she and Lewis well staffed? Wide-eyed, she laughs that much harder and in a Jean Brodie voice replies, "I am the staff! And Manon enjoys pottering around doing things with me. I think it's important for kids to see and understand that when you want a meal you have to cook it and you wash up and not everything is just presented to you in life. There are a lot of little lessons that can be taught around the home without sitting a child down and boring them to death with your philosophy of life!"

Her father's diplomatic career took the McCrorys around the globe, so hers was a childhood full of breathtaking vistas and constant change. Growing up, she lived in Norway, Nigeria, Cameroon, Zanzibar and France. Of all the spots they inhabited, which haunts her most vividly?

"Tanzania! It is so beautiful. I still remember seeing the herds of elephants and wildebeests and giraffes. I want to take my children there, if we can afford it, but because they're so young, I'd wait to take them when they'll remember it and will be incredibly grateful to us, and not waste our money now! Also, the Tanzanians are wonderful people. Considering they're living in a country with such hardship, they have an amazing optimistic outlook and happiness. Whereas here we have far more and we're grumbling and miserable."

I'm intrigued by her children's names, which are gloriously unique without being "Apple Martin" bonkers. Manon, stress on the first syllable, is Welsh, in honour of her mum and Lewis's dad. "Gulliver came about because Damian was working with an actor who was in Gulliver's Travels and we both really liked the name. When we phoned people up there was a stunned silence on the end of all the calls. But it suits him, because he is a giant amongst men already. He's also got the name Cameron, from my father's mother's side, to get the Scottish in, because he doesn't have McCrory. Gulliver Cameron Lewis sounds like a Victorian explorer or something. I think it's a great name."

Though she hopes her kids' passports will boast plenty of stamps, and the aforementioned British negativity notwithstanding, McCrory is passionate about London and happy to be home. "Don't say anything against it because I'm in love with London, particularly because I've been away for a year (while Lewis filmed the American television series, Life]. I think change is good because it teaches you that it's nothing to be frightened of. If you can do that with children when they're young, and in a safe environment, that means it's something they look forward to. I've never seen moving around a lot as a disadvantage because you're constantly going to be in flux, whether you're living in the same village or not."

Apart from Lewis's gruelling schedule ("You Americans sure can work," she virtually accuses), the year in Los Angeles was blissful. She did a bit of acting on Life but was mainly there as a wife and mum, hanging out by the pool in a pair of pyjama bottoms and a bikini top, enjoying her kids.

Earlier, she insisted she was uneducated, suited only to repeating others' great lines rather than composing her own. She even suggested I must be disappointed to find her so dim – which is amusing not only because most actresses labour mightily to prove themselves smarter than they look, but because it's patent nonsense. For instance, she tells me, "One of the lovely things about LA was having time to sit of an evening and read. I love reading. And I always read around a part that I'm playing, which is fun, but it was lovely reading for no other reason than enjoyment. I read F Scott Fitzgerald, and I'd always wanted to read a lot of Dostoyevsky."

It's my turn to laugh. For someone claiming she's not too bright … "Having said that, when you go to those old masters you realise that the reason they're masters is because they're accessible. That actually, The Idiot is very funny and a real page turner and that's why it's lasted. If it was just incredibly turgid and dense only four academics would have read it, and I'm not an academic."

That year in LA was partly facilitated by Gulliver and Manon's portability. Is there a master plan for when they're a bit older? "The children shouldn't be alarmed by this, but there is no plan whatsoever. It's likely we'll raise them in Britain, because of family. One of the exciting things about going to Los Angeles was that it's a great adventure, and especially when you're newly married with two young kids, you feel 'We can do anything now,' because you suddenly realise that you're a unit and that you have everything you want. As soon as you got off the plane and went to the hotel, you're there. It doesn't matter if you've unpacked; your husband's there, your kids are there, that's it.

"That's amazingly freeing and also, very romantic, when you've just got married; it's a huge adventure."

But it wouldn't do for the long term, she says. "LA was the most alien culture I'd ever lived in. I don't want my daughter to grow up in a city where looking sexy is so important. I had friends whose kids were seven and eight, wanting to go on diets. That was never part of my childhood and as a feminist I would never want that to be part of my little daughter's. People always say, 'Isn't she pretty?,' to a girl, and I'm like, 'Yes, she's very clever; she runs very fast'.

"Luckily they have a mother who (demonstrates] it's obviously not that important what you look like!" Puhlease, lady, pull the other one, I say. "Listen, I may have brushed my hair today, but it's because it's Friday and I'm going out tonight and I know I'm going to have my photo done. I'm very sloppy around the house. In LA, wearing Uggs was dressing up. I am so low-maintenance at the best of times but that really suited me to a tee. But you also show by example, don't you? You can't say to your child it really doesn't matter and then be neurotic."

In addition to her role as Narcissa (I'll forbear from the cheeky transitional sentence I'm itching to employ here), McCrory will play Cherie Blair again, opposite Michael Sheen's Tony, in The Special Relationship.

"I've never revisited a character before. Also, the first time, when we did The Queen, there was hardly anything on Cherie, because Blair was still Prime Minister and she was very, very discreet, in the background, not being interviewed."

My face contorts with disbelief and she raises an admonitory finger. "I fall in love with my characters so don't you start with your personal views!" But surely McCrory remembers the outspokenness and the famous gaffes?

"Yeah, but I'm never sure if she makes a gaffe or does it out of wickedness and then goes 'Oh, I didn't mean to say that.' Now, of course, she's written Speaking for Myself. Last time I Googled Cherie Booth QC there was lots written but it was all hearsay, which isn't very good when you're playing someone. This time there's going to be a lot of stuff, which makes it more difficult. Ultimately, though, it's not my Cherie Blair, it's (writer] Peter Morgan's Cherie Blair. I don't select the scenes; I don't select the way she's going to be portrayed. It's always about serving the piece or serving the director."

So McCrory never disagrees with her directors about a character's portrayal? "You have your script and that's your starting and ending point. That's the way I work. I was alarmed when I used to find actors who'd say, 'I want to change that line, I think my character wouldn't say that.' You think, 'Well, she clearly f***ing does because it's on page 17! You know your character better than the writer? I don't know what Pinter's going to think about this!' I always thought if you didn't understand why you said that line, go away and work it out. That's the job."

This film centres on the relationship between Tony Blair and Bill Clinton during the four years encompassing Stormont, Kosovo and the Lewinsky scandal. It features Julianne Moore as Hillary and Dennis Quaid as Bill. "How fabulous is that? You look at him and you're like, 'Yeah, you did 300 interns!' "

It's time to dash for the airport. During rushed goodbyes I neglect to tell McCrory that she's not the only one who falls a little bit in love with her characters – it happens to journalists, as well. sm

- Harry Potter and the Half-Blood Prince is in cinemas on Wednesday 15 July.