THE flashy, glitzy American gambling capital Las Vegas encourages people to let their hair down and do things they might regret in the morning. That’s certainly what happens on the screen, as surprise comedy hit The Hangover sees a bridegroom and his friends suffering – enjoying would be the wrong word – an event-packed bachelor party, or stag night as we Brits call such booze-fuelled occasions.

The revellers wake up in their hotel after a night on the town to find a baby in the cupboard, a tiger in the bathroom and no sign of the groom. The day goes from bad to worse, with an acrimonious meeting with boxer Mike Tyson and an encounter with a naked Asian man.

As The Hangover opens in cinemas, Sky Movies Premiere screens a pair of Vegas-set movies. What Happens In Vegas stars Ashton Kutcher and Cameron Diaz as strangers who say “I do”

after only a couple of drinks. The drama 21 has maths professor Kevin Spacey taking his students on a gambling trip to Vegas with the aim of beating the system.

The Hangover star, Justin Bartha, who was in two National Treasure adventures, and plays the missing bridegroom, reckons Las Vegas would be the last place he’d choose to throw a bachelor party.

Co-star Bradley Cooper’s only experience of a stag do was playing golf in Palm Springs with four friends. But he can see why Vegas has the reputation it has.

“If America is the machine, Vegas is the one place you can unscrew the valve and let off some steam. Las Vegas is the playground for the adults,” he says.

“Sin City, Sin City,” adds Bartha, warming to the topic. “An oasis of debauchery”

in the desert.

But the actors – Ed Helms and Zach Galifianakis who play the others in the group – didn’t emulate their characters by partying hard when the cameras stopped rolling. “The truth is we worked 16 hour days,” explains Cooper.

“We lived in Caesar’s Palace hotel, but we made a war movie. We got the crap kicked out of each other. We were in the desert all day, so when we wrapped filming, how were you going to wind up the day? Get drunk, go to a strip club? I’d just been mauled by a tiger and had an Asian man’s genitals flapping round my neck.”

Helms, whose dentist character wakes up the next morning to find he’s missing a tooth, but has gained a stripper bride, takes the same line. “We were working so hard – up at 6.30 in the morning and in bed by 9.30 at night. All the exciting stuff happened on the set,” he says.

It wasn’t possible to top what they were acting out for the cameras. Bartha didn’t even have any experience of bachelor parties. He’s never been to one, he says.

“I’m an actor, so I get to do this stuff and get paid for it, so I don’t have to do it in real life.”

Galifianakis reckons he’s not much of a bachelor party guy either. “I don’t need an excuse for celebrating, to get s*** faced,” he says.

Few people in Vegas paid much attention to the antics of the film crew. Bartha recalls walking around the lobby of the hotel with his sunburn make-up on, wearing a robe for hours and no one paid him any heed.

But could anyone smuggle a tiger into their room as happens in the story? Cooper doesn’t doubt it. “You could walk an elephant through the lobby and no one would care. Vegas is completely indifferent to anything that happens,” he says.

The whole movie felt real because they were living in Vegas for six weeks during filming. Returning recently for a four-day junket to promote The Hangover wasn’t a worry.

“Vegas doesn’t scare me any more,”

says Cooper, who’s tipped to play smoothtalking womaniser Lieutenant Templeton “Faceman” Peck in the forthcoming A-Team movie.

“Being there,” says Bartha, “you feel you’re in a show and can’t get out of it until you leave. Once you get to that Vegas strip, the energy just changes. It never stops.”

Viva Las Vegas, as a certain Mr Presley said in his movie set in that city.

■ The Hangover (15) is now showing in cinemas.

■ Wedding Crashers: tomorrow, Film4, 9pm.

■ What Happens In Vegas: today and tomorrow, Sky Movies Premiere, 8pm.

■ 21: today and tomorrow, Sky Movies Premiere, 10pm.