When Ashton Kutcher was living the high life, he knew how to have a good time. He once managed to transform the mundane-sounding Annual Video and Software Conference into a night of wild indulgence.

“That was an insane weekend,” he recalls. “I was there in Vegas to collect an award, and found out that the Adult Film Awards were happening the same night. I turned $1,000 into $25,000 playing craps, then ended up on stage with a rap group, with a bottle of champagne in one hand, standing next to Ron Jeremy, rapping to a sea of porn stars and silicone.” He smiles. “The rest of the night is fuzzy.”

The 30-year-old star is in the UK with his wife, the 45-year-old actress Demi Moore, to promote his latest film, the romantic comedy What Happens in Vegas, opening next week. He and Cameron Diaz play strangers who share a booze-fuelled night of revelry in Sin City.

Before he met his wife, Kutcher belonged to the rapper P. Diddy’s self-styled Rat Pack. Surely he knows about tearing it up in Vegas?

“I never went to Vegas with Diddy,” he says. “But we did hit the Atlantis Casino in the Bahamas once. The story from that night is not for sharing. That’s . . . definitely a ‘BD’ story.”

A “BD” story, it transpires, is a tale from his life before he met Moore. “We’ll be at the dinner table, I’ll be telling a story and suddenly Demi will go, ‘That’s a BD story’. It’s ‘Before Demi’, so she’ll put that disclaimer in straight away.”

Kutcher’s life “BD” is brimming with “enough debauchery to last a lifetime”. On the night that the couple met in 2003, he was partying in Los Angeles, where, having split with the starlet Brittany Murphy, he was eyeing up women and knocking down cocktails. He had much to celebrate. That weekend he had hosted Saturday Night Live, while his MTV show, Punk’d (a celebrity version of You’ve Been Framed), hit the top of the ratings. He was already starring in the popular kitsch sitcom That ’70s Show.

“Not long after I met Demi she wanted to move back to LA from Idaho,” he remembers, “and she came to stay with me. So in my bachelor pad was my room-mate, Demi, her three girls and me.

“For a couple of months, at midnight the doorbell would ring, and a car full of girls would stop by to say hello. My roommate . . . intercepted most of the notes and stuffed animals, but, occasionally, Demi answered the door. That didn’t go down too well. I remember people thinking we were together for commercial reasons, but I don’t think I gave a shit. My career was doing fine when I met Demi.”

In 2005, the couple married, just as Kutcher was voted the world’s most eligible bachelor by Tatler, beating Prince William to top spot. Some have suggested that only an older woman could have tamed him but Kutcher dismisses the notion. “It’s got nothing to do with Demi being an older woman,” he says. “She’s not someone who is ‘been there, done it’. You might think that, but . . . she still sees the world every day, like something new, and that’s why I find her so attractive.”

Kutcher was born in Cedar Rapids, Iowa, and says that his love of acting was prompted by a childhood beset by insecurity. His parents were factory workers, money was tight and his twin brother, Michael, had cerebral palsy.

“I had a lot of natural insecurity,” he says. “I was poor – financially, not spiritually or emotionally – and I was small.” The one place he felt happy was on the school stage. “I remember my first school play, The Crying Princess and the Golden Goose, and I walked out there with the intention of making people laugh. And they did. That’s when I first got real confidence.”

That confidence blossomed when Kutcher entered his teens, and while he has painful memories of his parents’ divorce, he remains grateful to his stepfather, Mark Portward, who was always supportive. That experience, he says, has helped him to forge a good relationship with Moore’s three daughters, Rumer, Scout and Tallulah, and he maintains a friendly relationship with their father, Bruce Willis.

“It helps that they are girls,” he says, “because if they were boys there’d be some competition. But because they are girls, they compete with their mother. It’s not like I get to be the best friend, mind you. I just guide them in different ways.”

Kutcher hopes that his forthcoming films will help to establish his credentials as a serious actor; he is aware that a CV filled with films such as My Boss’s Daughter – in which his character’s most memorable moment involves stuffing pills up an owl’s anus – has not showered him with credibility. After What Happens in Vegas he will star in two dramas, Personal Effects, with Michelle Pfeiffer and Kathy Bates, and Spread, with Anne Heche and Laura Linney.

“I would just caution people: don’t doubt me,” he says. “When you see those movies coming, don’t just dismiss them; just give me a shot.”

What Happens in Vegas opens on May 9 2008

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