The gag is that a glossy wedding magazine is running a competition to find the most over-the-top marriage ceremony.

A shortlist of three couples is selected and wedding planners are provided to help them make their dream day come true. The first want to turn their big day into a lavish Busby Berkeley-style musical in which the climax comes when they sing their vows to each other. The second plan to say 'I do' in the middle of a tennis match. To the horror of the magazine editor, the third couple . . .chosen by a celebrity jury . . . are naturists who are determined to go to the altar completely naked.

The fun comes from the fact that right from the start Issit isn't the slightest bit coy. It becomes so unselfconsciously natural that you almost stop noticing the nudity, and just laugh at the social incongruity of a bride and groom who most of the time aren't wearing a stitch of clothing.

"At the end of the day, what's all the fuss about, " says Issit. "They're just naked bodies."

She got the idea while researching weddings on the internet. "I hit upon a site that said 'Naked Wedding'. Brilliant, I thought. So we moved into a naturist colony to find out what it was all about. We all had to take our clothes off and join in what they were doing. They were a bit nervous in case we were just going to send them up. It suddenly became really important to me to give naturism a positive spin. Because although Confetti is a comedy and we want people to laugh, it's all very innocent even if some people might think it a bit rude."

Confetti is being acclaimed as the next big British comedy hit after The Full Monty and Four Weddings and A Funeral, although how it plays in the US may be open to doubt. "The attitude to screen nudity is completely different over there, " says Issit.

"They show hardly anything, on the telly or in movies. They're very strict. We did a test screening in a shopping mall. The majority of the audience by the end of the film came round to the idea that it was okay. But there were some who said it was 'unchristian'."

She has no intention of fudging the nudity to placate prudish fundamentalists.

"It's an integral part of the movie, " she insists. "If you cover it up or cut it out, there's no story. The story hangs on the idea that there's nothing wrong with it."

The nudity is not all that's unconventional about Confetti. It was filmed live with all the actors improvising their characters. "They were absolutely committed to the idea of making it up as they went along, " she says.

"I just set them a wedding date. I said you've got six weeks to plan your weddings.

And everything that happens, happened on camera. There was no rehearsing. The whole point is that it had to be genuinely spontaneous. We filmed the wedding day as a live event. It was the first time I'd seen those weddings myself . . . and I'm the director. I just set the whole thing in motion and then sat back and let it play out. I just made sure the conditions were right so the fall out would happen."

Issit wants to loosen up the process of filmmaking. "Making films is very slow and very boring. They've become too technical.

It just doesn't suit my personality. The actors have to sit around in caravans for hours. Then they come on and they're expected to sparkle. For me it's like, let's put the actors centre stage as we do in the theatre and get the crew to run after them."

It's an approach that appeals to Jessica Stevenson, the TV comedy star who plays the role of a tone-deaf singing bride.

"Debbie just has confidence in the acting process, " she tells me.

"She understood that if she created the right environment and then just set us off, we'd come up with the goods . . . which somewhere in the 150 hours of footage she shot, we did. We knew where we were going but lots of times we didn't know how or even if we were going to get there. Debbie wasn't even sure we were going to get married at the end of it all, but I think we were determined that we would."

Confetti has the spontaneous feel of a Mike Leigh film or of sitcom The Royle Family, in which Stevenson . . . who collaborated with Simon Pegg on Spaced and Shaun of the Dead . . . played the role of the neighbour Cheryl.

"Mike does all the improvisation in rehearsal where it's locked down into a script which he then films, " she says. "It's never been done live as we did in Confetti.

There's no improvisation at all in The Royle Family. It's all scripted, right down to the pauses . . . like Harold Pinter. It's absolutely by the book, all in the text. Yet although it's completely different to Confetti, funnily the results may seem similar."

As Altman demonstrated so uproariously in A Wedding, marriage particularly lends itself to improvised comedy. "There's something for everyone to identify with, " says Stevenson, who has just given birth to her third child. "I suppose both Debbie and I drew from our own experiences of family conflict. Most people have rifts in their family that come to the fore at weddings."

Issit comes to movies after 20 years theatre work with the Snarling Beasties theatre company. Her movies, like her plays . . . notably The Woman Who Cooked Her Husband . . . invariably focus on commonplace domestic relationships. "It's all very accessible stuff with a bit of black comedy going on, " she says. "Just stuff you feel around you everyday. It's so simple but so sad. And everyone can relate to it.

There's too much pressure on filmmakers to make big epic movies."

She got the idea for Confetti while daydreaming about what it would be like to have a musical wedding. "I grew up with all those old musicals of Gene Kelly and Fred Astaire. My mum made me watch them. I loved them. It's something secretly I'd quite like to do myself."

Nobody in her family acted or wrote plays, but as a small child she was steeped in the ambience of theatre. "All of my aunts and cousins were usherettes and have been for years at the Birmingham Repertory Theatre. Since I was two I've seen nearly every production there . . . they snuck us all in. So we sort of grew up with the theatre idea."

She trained as an actor and then started writing because she couldn't find anything that she wanted to be in. "Although I admired the classics, I was always a contemporary person, but there were only bit parts for women in most of the modern plays. So I wrote myself a lead part. Then I thought I better direct it because who else is going to cast me. So it grew from there and I'm still doing that, basically."

Like the way she works, her career has evolved spontaneously. "If you want to make God laugh, tell him your plans, " she says, quoting an old stage dictum.

"I began making films because I was fed up having to tour all over the world night in and night out without having anything to show for it. All that was left afterwards were the programmes, which didn't sum up what I was doing. I wanted to have something to show for it."

After a couple of shorts she directed a movie version of her black comedy Nasty Neighbours, starring Ricky Tomlinson. "I felt I should start with something I was familiar with, before trying anything original."

Confetti was filmed in six weeks, but took 63 weeks to edit. "It's a shame so little of what we shot could go in. There are hours and hours of Alison Steadman, who plays Jessica's mum, and when I watch it at home it's like a one-woman show. And it's the same with Martin Freeman and Steve Mangan and all the others. I felt their blood on my hands as I cut all their wonderful stuff out. I really hope Confetti works on some level. If it doesn't, we'll never get a chance to work like this again. None of us will get any money from it. It's all about, can we be playful like this again?"

She feels the British film industry is too cautious.

"It's all about ticking boxes. Every time they invest, they want a certainty. So much of it is like betting on horses. You're never going to get rich just putting your money on favourites. It's always the long shot that pays off in movies. I don't long to work with Tom Cruise, but with people who are down to earth, creative and real. Movies have become too much about Hollywood stars and not enough about life."