It’s so dull when the first line states that the star arrives wearing “not a scrap of make-up”. I could pretend that Natalie Portman had false eyelashes that fell into her tea and hot-pink lipstick smeared across her cheek as gawpers in this Santa Fe cafe turned to stare. And the great thing is, she’d probably play along.

I first interviewed Portman, in her home town of Long Island, when she was 16. Now, 10 years on, she is as sweet – and clear-faced – as I remember her. Back then, after the interview, we almost collapsed in K-Mart when we found promotional toothbrushes bearing her Star Wars image. Today, in Santa Fe, where she is shooting Jim Sheridan’s Brothers, we shop at Wholefoods for ingredients to make vegan cupcakes with the actors who play her and Tobey Maguire’s children. Yes, the girl who became a star in Leon, aged 12, is playing a mum. “I knew how to act like an adult from a really young age. I could turn it on and off. They’d say, ‘She doesn’t seem like a kid, she seems like an adult.’ Now, it kicks me in the butt, and people think, ‘She doesn’t seem like an adult, she seems like a kid.’ ”

Portman has close friends who are having kids now, such as the flame-haired actress Bryce Dallas Howard, who, when they were teens at summer camp, played Helena to her Hermia in A Midsummer Night’s Dream. Portman feels a while from motherhood herself, though she is in a serious relationship with Nathan Bogle, the British model and a founder of the Rag & Bone label, whom she politely refuses to talk about. She has consistently declined to talk about any beau, but does say that, as an only child, “I always want to be with someone who has siblings. I will never date another only child”. She pauses. “But I think I have dated exclusively first children.” Harvard, which she famously dropped out of Hollywood to attend, is “70% only and first children”, she says, “which makes sense. My career would not have happened if I had brothers or sisters. There’s no way my parents would have allowed one kid to be a star”.

Although quite a few films are now made in this artsy New Mexico town, Portman is the biggest thing to happen to Santa Fe in some time. To begin with, there’s that face – nose, eyebrows, cheekbones and chin of glass – paired with round, soulful eyes and the softest mouth. Because of that face, because of the slender lines of her body, she has the reputation for being the most elegant of young actresses, and, yes, she has a jade cashmere sweater, skinny blue jeans, a check coat that ties smartly at her tiny waist. But she’s also wearing big, green, deeply inelegant wellies, because she has found the cheaper a shoe is, the more likely it is to be vegan. Hence her launch of a shoe line for Te Casan, with all profits going to the Nature Conservancy.

“Basically, I did it out of a lack of choice. Stella McCartney does great shoes, but they’re expensive and very fashiony. I wanted a mary-jane shoe without leather. I’ve been getting stuff from Target, which is de facto vegan because it’s so cheap. But I did need some shoes that weren’t made of canvas or plastic.”

With values instilled by her American artist mother and Israeli doctor father, good deeds are par for the Portman course. Producing the first-ever iTunes charity album, she raised an enormous amount of money for the Foundation for International Community Assistance (Finca), which provides micro-financing for the world’s lowest-income entrepreneurs.

Last year, Portman went to Rwanda to make a documentary about endangered gorillas for the Discovery Channel. This year, she begins a deal with Participant Productions, the makers of Syriana, which has an ethical mandate to make films that promote social change. Her first movie as director will be an adaptation of A Tale of Love and Darkness, Amos Oz’s history of his family and Israel. The scope is vast, and Portman, who was born in Jerusalem, is glad she did six months of college there, if only because it brought back her Hebrew.

It’s hard to imagine that in 2002, she co-hosted a new-year party with Britney Spears. What on earth did the two have in common? “We both had the same first job. We both understudied for an off-Broadway musical called Ruthless. I never met her, but I took over from her when she left to join the Mickey Mouse Club. So we connected over that. She invited me to a few things and my guy friends at college said, ‘We have to go.’ I’m sad to see how everyone’s treating her now.”

Now that Britney’s star has fallen and she is mocked as coming from poor white trash, does Portman think there are also stabilising advantages in belonging to an ethnic minority?

“Absolutely. I identify very strongly as Jewish, but I could be Indian, Puerto Rican . . . Anything that gives you a cultural identity makes you know who you are and grounds you, even as a young girl trying on identities.” She sighs. “Any time I see something about Britney, I close it. I can’t look at it. I’m usually interested in gossip, but this makes my stomach hurt.”

She feels extremely lucky to have missed the worst of the paparazzi madness, with the videographers and bloggers. “I was allowed to establish myself when it wasn’t this crazy. I was going to high school and college and there were no paparazzi after me. But that was only five years ago. It wouldn’t happen now.”

One such piece of tabloid bait is Scarlett Johansson, with whom Portman shares the screen in The Other Boleyn Girl. They play sisters in a love triangle with Henry VIII. “It’s a catfight movie.” Highbrow catfighting? “Not really highbrow,” she laughs. “Have you read the book?”

She catches herself, then does her best to think her way into promoting a movie that is getting mixed reviews. “These two sisters love each other so much, and are made out to be rivals by the values of society that uses women as pawns.”

She doesn’t want us to think the film fosters rivalry between women. “I hate that – it drives me crazy. The truth is, it’s hard to find a really great girl, but when you find one, it’s the best. The vast majority of my friends are guys, but the ones I talk to about everything are my girls.”

Back when Portman was 14, she was the face of Isaac Mizrahi, who would go on to cause a minor scandal at the Golden Globes two years ago, when he grabbed Johansson’s breasts. “Seriously, I would want to grab them. She’s got beautiful ones, but, Isaac, that was not so appropriate.”

And, of course, while Portman is famously Jewish, Johansson is a lesser-known Jew (because of her Scandinavian father, she’s called “the kosher Danish”). When Penelope Cruz and Salma Hayek made a movie together, all the headlines blared “the Hot Tamales”. What should the media label a film starring two Jewish girls? Portman doesn’t miss a beat.

“The Hot Knishes,” she says, referring to the Jewish delicacy. She says how good Johansson is, adding that in her own forthcoming film, My Blueberry Nights, Rachel Weisz “will knock your socks off”. But her favourite co-star of all time?

“Stephen Fry, when we made V for Vendetta. I wanted to spend all my time with him. He is awesome.” She thinks they gelled because they have anachronistic personalities. “I would much rather read a book or write a letter than watch television or send an e-mail. And I value good spelling. Spelling errors are a total pet peeve, which is embarrassing.”

Not that she is the paragon of refinement that her face would lead you to believe. “My Hallowe’en costume the last year of high school was a hooker,” she admits. An elegant, gamine hooker? “No,” she laughs, tea dripping onto her cashmere. “Just a hooker, with lots of bad make-up.”

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