It’s 8.30pm on a cobalt-sky evening in Barcelona. Penelope Cruz floats through the bar at the Hotel Arts and settles on a brown velvet sofa. In her ethereal white dress and skimpy sandals, she looks like a classical goddess. It’s hard to know which bit of the divine countenance to focus on first: the faultless bow of a mouth, the dark cascade of hair, the compelling ebony eyes set beneath arching brows. She flashes a wide smile – friendly, but wary – and reaches for the bowl of nuts.

I ask her about the dress. “It’s from the movie,” she says, her English cadenced with elongated vowels. “I lost my luggage, so they gave me clothes from the character.” Cruz is in Barcelona to shoot a new Woody Allen picture with Scarlett Johansson, currently known simply as “the Spanish project”. She won’t discuss it, apart from saying that working with the director “is a huge thing for me”. But then, she’s not here to talk about films.

Together with her sister, Monica, a dancer and television actress, Cruz has designed a collection for the Spain-based fashion chain Mango. The range, a mix of red-carpet-inspired evening wear, jeans, knits and daywear, is based on vintage pieces from their own wardrobes. Monica, 30 to Penelope’s 33, looks unnervingly like her older sister (she once even appeared on a television show as a Penelope-alike), and together they have a combined sexual voltage that could overload the national grid. It’s like a seriously exotic identical-twin fantasy.

The sisters are proud of the collection, which will be launched at a party in Ibiza this month. “This is something we’ve wanted to do for a long time,” Cruz says. “We just had to find the right people to do it with. We’ve always loved clothes – we used to hide in the bathroom with fashion magazines and say, ‘I want that one.’ We’d get a pencil and mark the pictures, and show how we’d transform them at home.”

The Cruz sisters grew up in the working-class suburb of Alcobendas, north of Madrid, the daughters of Eduardo, a retailer, and Encarna, a hairdresser. There is also a younger brother, Eduardo, who looks like a cross between Ricky Martin and Enrique Iglesias, and is building a career as a singer-songwriter. Both sisters have said they are keen to dance on his next video.

Family is clearly important to Cruz: ask what she most cherishes, and she glances at her hand and turns her grandmother’s ring. “My family are the best thing I have, and I spend a lot of time with them,” she says. “We are very strict about that, about not letting anything interfere with having time for the family. We are always here for each other, all of us. We know that we can count on the rest, so we always find the time.”

She credits her parents with her drive to succeed. “I’ve always been a perfectionist, to the point where some people can misinterpret that as being a control freak. I put more time and more energy into something because I want it to be as good as possible. And my sister is the same. She is very . . .” she grasps for the word “. . . a very hard worker. That’s how we grew up. My parents taught us that you earn every day what you get. They didn’t have a lot of money and they had to work really hard to raise us, so we feel like every opportunity we get, we want to give 100%.”

Cruz still has a home in Spain, but her burgeoning film career necessitated a move to LA – a city where, she says, she initially felt uncomfortable, but that she has grown to love. She is now a central fixture of the Hollywood scene: last month, she was singled out by Prince to be serenaded when she sat in the front row at one of his concerts. She has dated – or is rumoured to have done – a string of leading men, most famously Tom Cruise, with whom she had a three-year relationship after they met filming Vanilla Sky. They have remained friends; Cruz was reportedly among the first to see Suri, his baby with Katie Holmes. She then spent a year with Matthew McConaughey, and has been linked to Orlando Bloom, Josh Hartnett and the rocker Lenny Kravitz. After a jokey picture of Cruz pinching Salma Hayek’s bum appeared in a magazine, the gossip sites even ran ridiculous rumours that the pair were having an affair.

She says she has “a circle of dear friends” in LA – including Hayek, Demi Moore and Ashton Kutcher. A few years ago, Cruz helped to set up Hayek for Kutcher’s celebrity prank show, Punk’d. The scenario was this: the loo in the restaurant is backed up. Cruz takes Hayek to show her, and they laugh at how disgusting it is. When the loo overflows, the manager blames Hayek. Cue outraged reaction and embarrassed denials. “I love to laugh,” Cruz says. “Ashton came up with the idea and said, ‘I’m sure you wouldn’t want to do something like this.’ And I said, ‘I’d love to do that to one of my best friends.’ It was very funny.”

With her time split between Spain and the States, you wonder if it feels as if she is living two lives. “No,” she says. “My lifestyle in LA is similar to the one here. I like doing things at home – you know, long dinners with my friends. I don’t like too much nightlife in places where you can’t talk, unless it’s to listen to music. I just fly with music. I don’t need anything else. It’s such an important thing to me.”

I tell her I have heard she is a bit of a karaoke addict. “I have a machine at home,” she confesses. “Sometimes I do karaoke parties: everyone arrives saying they’re not going to sing, but everybody sings.” Do you force them? “No, they all say, ‘I’m too cool for karaoke’, and then they are all fighting for the microphone.”

She laughs, and for a moment I feel like I’m glimpsing the mischievous funster behind the polished international star. I ask if she would describe herself as a girl, a woman or a lady. No hesitation: “A woman.” It’s true that the youthful innocence she had in her early films has been replaced by a more grown-up, sophisticated sexiness.

Ah, yes, the sex appeal. It is something of an occupational hazard.

Stephen Frears, who directed Cruz’s first English-language film, The Hi-Lo Country, tells of the moment he first saw her on a video: “I went and looked, and there was Venus.” Some of her best-known roles have been charged with sensuality – not least Raimunda, her character in Volver, the Pedro Almodovar film that earned her an Oscar nomination. At times, the camera lingers indulgently on the voluptuousness of her cleavage and her bum, which was prosthetically enhanced for the film.

But Cruz doesn’t want to be simply the Spanish one who looks hot. “I’ve been lucky to play so many different roles,” she says. “In the Italian movie Don’t Move, I had to be totally deformed. People made a big deal out of that, saying, ‘How come you weren’t scared?’ And I said, ‘Because that’s how my character looks.’ I’m there to serve my character, not my ego.”

Still, Cruz stars in this year’s Pirelli calendar alongside Sophia Loren, Hilary Swank, Naomi Watts and Lou Doillon. That, surely, was a celebration of her own sensuality, not just playing a role. “It was one day of pictures with two photographers I really admired,” she says. “Sophia Loren left me a note. We weren’t there on the same day. We all had seen the ones they’d done before, and they were very beautiful,” she adds, smiling again. And the pictures she and Monica did for Mango aren’t bad either.

Penelope and Monica Cruz for MNG limited-edition collection will be in store from September 16; www.mango.com

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