What on earth has happened to Scarlett Johansson? The poised young woman in pin-neat Louis Vuitton and a hefty inch of make-up, on the sofa of a London hotel, is barely recognisable as the carefree indie princess who, so they say, used to have sex in lifts. She’s not even blonde any more.

“Oh, it’s just a hair colour,” sighs Johansson, raking a manicured hand through her new cherry locks and curling her curves into an inky black question mark. Her voice, a surprisingly mannish baritone, sounds like Velcro being slowly peeled back. “I’ll be red, I’ll be blonde, I’ll be black. When you work, you maintain the same look for a whole period of time, and when you’re finished you want to cut your hair off, change something about yourself. Maybe that was a little bit of it.”

She’s still staggeringly pretty, of course; petite — 5ft 4in on a good day — and neat; and far more striking in the flesh than Keira or Sienna, although I’m surprised how thin she is, in spite of her shape. Her waist can’t be more than 23in. She’s here on a flying visit as the new “face” of Moët & Chandon champagne, a slightly bizarre collaboration in which she will be wheeled out as the new Moët muse at a party in north London. Her handlers have sent me an e-mail stating that Johansson won’t answer any questions about her private life, but is happy to talk about her love of champagne. The trouble is, she’s not even much of a drinker. But when so many other Hollywood stars are selling out to luxury brands, I guess it was only a matter of time before Johansson cashed in, too. She also has contracts with L’Oréal and Dolce & Gabbana. She’s not an indie ingénue any more. She’s a business.

Hers is a familiar A-list career trajectory: credible starlet with the figure of a siren becomes a global commodity. Soon, mainstream films come knocking (Iron Man 2 is her next project), and everyone else wants a piece of the very profitable action. “I have a healthy ambition,” is how she puts it. Quite: a friend who met her a few years back described her, at 21, as “unnervingly self-assured, knew exactly what she wanted and was financially savvy”. Today, she has her “business face” on, and her answers are appropriately perfunctory and vague. She’s surprisingly unfeisty, too, perfectly happy to accept her stereotyping as a dumb blonde, in spite of her obvious intelligence. “In this day and age, everyone’s thrown into a category,” she shrugs. “You’re either dark and mysterious, or blonde and pouting, or you’re brainy and the girl next door. It’s hard to not be put in a corner. But as you get older, you have a different range available to you.”

But doesn’t the constant scrutiny of her looks get a bit irritating? A while ago, the global obsession with her breasts reached the point where the fashion designer Isaac Mizrahi actually grabbed them on the red carpet. (Johansson was wearing spray-on Valentino: they did look spectacular.) “That was horrible. I didn’t know what to do. It was very awkward,” she says. “It’s funny, coming up in the industry: I never thought about being sexualised. But people are obsessed. Obsessed with weight, curves. It’s nuts. I have my own standard of how I like to look. I’m a healthy person. I take care of myself.” And, of course, there is life after breasts. “I really do see my career going into film production and direction,” she says.

Born in New York into a theatrical family, she had a childhood that was “normal — I lived in a neighbourhood, went to a normal school”, except, of course, she was also a child star, who shot to fame at 13, as the girl with the stumpy leg in The Horse Whisperer. She has now been in the business for 16 years. She describes the best part of her job as “the magic of movies”, an expression that leaves me slightly cold, especially when you consider the uninspiring Iron Man 2. But, hey. A girl’s got to earn a crust.

Last year, it was a bit of a surprise when she quickly settled down and married the actor Ryan Reynolds, who strikes me as a bit of a Hollywood himbo. After Josh Hartnett and Jared Leto, surely she’s more of a River Phoenix girl. Reynolds got his movie break in Blade: Trinity.

“It’s private,” she says, snippily, but is slightly thrown when I tell her I might not be interested. “Why do people wanna know the things you don’t really wanna tell? For me, I’m much more interested to read about the process of how they got into character, because I’m an actor.” So, is that why she posed nude on the cover of Vanity Fair? “That to me was different,” she says. “I knew it would be a beautiful portrait. That’s not to say your body is not sacred; if it was someone groping me, or posing for some gratuitous magazine, that really doesn’t interest me. I’m a sensitive person. Hypersensitive. I don’t belong to anybody. F*** no! People fish for things, they’re very greedy about extracting information, so I hold those things very sacred, because that’s me. I don’t proclaim to be a role model for anybody. I’m just me.”

Yet she is a role model. Only last year, she caused a storm by admitting that she had an HIV test twice a year. “It became, like, she’s a tramp, she gets an HIV test,” she says. “At the time, I was single. You have to. It’s mandatory. Why would you not? People don’t want to talk about sex: it was a very strange reaction. Like, she’s having an HIV test! She’s having sex! At least it kind of starts the conversation.” As for the slanging matches with other celebrities, and the lift sex rumour, “You just have to swallow it,” she sighs. “It’s frustrating, but I’m not a very confrontational person. You hope it fizzles away.”

Perhaps she got married to put a stop to all of that. She admits she has been hurt by men in the past — “Definitely, for sure” — but insists there’s no perfect age to get married. “When you know, you go right ahead,” she says. “It’s not even about being ready to get married. You know when it’s something you want to do, a way of expressing your love. It’s a very romantic thing to do. For some people that happens many times. You hope not, but my mother’s remarried. Both times, it seemed right then.”

Tomorrow, it’s back to LA; she has been in London for little over a day. At least there’s a party tonight. “Yeah! Woo-hoo! It’s nice to let loose, especially after doing interviews. I know it sounds corny, but I’ve really enjoyed working with everyone at Moët.” But really, does she like parties she’s essentially paid to show up at? “Sometimes,” she says, carefully. “I usually invite friends of mine, so it’s not a whole group of people I don’t know.”

Later, I spy her at the party, surrounded by her entourage. A huge bottle of fizz appears at the front as smoke is piped into the room. The music crescendos as Johansson is swept awkwardly to the front. She doesn’t say anything, she doesn’t do anything, except flash a dazzling smile. The whole spectacle is as lavish as it is ridiculous, but if Johansson is embarrassed, it doesn’t show. When asked for her take on it, she flips into brand mode. “During these financial times,” she says, “champagne is an affordable way to celebrate, mark an occasion.” Attagirl.

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