When James Corden and Ruth Jones were sitting waiting to be weighed at Superslimmers - the fictional slimming class of Kay Mellor's TV series Fat Friends - they had no idea that, some years later, they would be writing their own comedy series and starring in it, too.
"We became friends during filming, and, when we were doing Series 3, James was talking about this idea he and his girlfriend had about a wedding that takes place between a guy from Essex and a girl from Barry Island", explains Ruth. "I thought it sounded brilliant, and we'd sit in the hotel bar where we were staying and look around, because there was always a function going on, and we'd pick out characters that might be at this wedding and improvise what they might say."
"I have ideas all the time and do nothing with them", says James. "But Ruth was adamant we should try and write this." After putting pen to paper, James and Ruth sent their script to Stuart Murphy, ex-Controller of BBC Three, who loved their idea.
"Stuart said he didn't have a slot for a one-off drama or comedy, but asked if we would be able to turn it into a series" says Ruth.
It was quite a scary prospect but James, in particular, received some encouragement from a very high-profile source - the author and playwright Alan Bennett. James was performing in Bennett's production of The History Boys at the National Theatre. "Alan gave me such confidence, and said to me: 'James, you should write some of these ideas down because they're really funny. There are lots of people who make people laugh now and again but you really have a chance, if you write these things down, to do something.' And when Alan Bennett says that, you can't really argue!"
In the space of three months, five episodes of Gavin & Stacey had been written - the first episode taking them just 24 hours.
"We wrote Episode 6 in New York while I was doing The History Boys on Broadway" James says. "Ruth came out to join me. It was weird sitting in a lovely apartment in New York trying to write a show about a regular guy from Essex and a regular girl from Barry! Sometimes we'd do some writing in Central Park which was great."
The end result is a sitcom about Stacey from Barry, near Cardiff, and Gavin from Essex. But the difference in backgrounds and culture doesn't get in the way of their budding romance, which flourishes when they finally meet for the first time after hitting it off during numerous flirtatious work calls.
James and Ruth play the couple's best friends, up-front Nessa and best man Smithy, a couple who couldn't be more dissimilar. Gavin and Stacey's romance may be a match made in heaven, but the same can't be said for Nessa and Smithy, who dislike each other from the start.
"Nessa is Stacey's best friend", says Ruth. "She has a very chequered past; has travelled the world three times; has worked in all kinds of places, including Harrods; and has had sex with most of the men in Barry, and probably most of the women, too! Nessa is very stoical and a really good friend, actually, in a strange sort of way."
Nessa's dress sense is somewhat brazen, so how did Ruth feel about wearing such bold clothes? "I think Nessa is very much stuck somewhere in the Eighties with black eyeliner and leather. But I kind of like that about her" she laughs. "I love any opportunity to change the way I look; it's great. The wig that I wear was actually the blonde wig I wore when I played Kelly in Fat Friends. It's been dyed and cut. I'm all for a bit of recycling!"
James's character Smithy, meanwhile, is Gavin's best friend. Friends since they were young children, their relationship is almost like a wonderful, platonic love story. One might think, therefore, that Smithy would be envious of some 'bird' muscling in on his best mate. But not so, according to James.
"I think Smithy would love Gavin to have a girlfriend, but not to have a wife", explains James. "In Smithy's head, they will move into a flat, be bachelors for about the next five years, get married at the same time, live next door to each other, have a couple of kids, take them to football, but still have a drink together. According to Smithy, that's how it should work, and that's the way you should do things. But, really, it's mates first and birds second. And, when Gavin says he's getting married, and as much as Smithy's pissed off, he knows it's right."
Gavin & Stacey boasts a stellar cast including Rob Brydon, Joanna Page, Larry Lamb, Melanie Walters, Ruth Jones, James Corden and Mathew Horne. There are also cameo appearances from comedy stars like Julia Davis, Matt Lucas, Sheridan Smith and Mark Wootton.
Gavin & Stacey also reunites James and Ruth with their Fat Friends co-star, Alison Steadman, who, in James's opinion, is one of the country's best actors. "We set out to write a great part for Alison, because we both knew her and both look up to her and idolise her as an actor", says James. "Alison's never anything but brilliant and we wanted her to play the role of Pam more than anyone. When she rang us and said she'd just read the script and loved it, we were over the moon!" adds Ruth.
"Once we'd started writing the part of Bryn, we thought Rob Brydon would be great. The minute he read the first two episodes, he rang Ruth and said: 'I really, really want to do it. I know exactly who he is; I know exactly what he wears; how he walks; how he speaks'."
Aside from Gavin & Stacey, Ruth Jones has played Peggy in the Bafta Award-winning film East Is East, and then went on to take a leading role in Nighty Night. She's perhaps best known for playing barmaid Myfanwy in Little Britain alongside Matt Lucas's 'only gay in the village' character, Dafydd.
Ruth also plays Magz, the girlfriend of Steve Coogan's character Tommy in Saxondale. She describes how she got the job: "I was at the Comedy Awards with Julia Davis, as Nighty Night was nominated, and Steve was on the table in front of me", recalls Ruth. "I'd only met him a couple of times to say hello to, but he turned round and said: 'You scrub up well, don't you? Will you play my girlfriend in my next series?' And that was that! It was the easiest job interview I've ever had!"
James and Ruth were overwhelmed when BBC Three commissioned their comedy series, although this soon turned to nerves in the run up to the show appearing on the small screen:
"You show it to your family, and they may be critical, but you just don't know if it's any good", says Ruth, sheepishly. James, on the other hand, feels as though they're taking a huge leap of faith: "It's terrifying in one aspect and hugely exciting in another. We certainly feel incredibly proud of what we've done, and we think it holds up and we think it's good."
They're not alone in thinking it's good - the first series was met very warmly by critics when it was transmitted in May 2007 - Radio Times called it 'one of the best comedies of the year'. The series has gone on to triple success at the British Comedy Awards, triumph at The South Bank Show Awards and pick up the 2008 BAFTA Audience Award. Quite a haul!
In March 2008, Gavin & Stacey returned to BBC Three for a second series, opening with a double episode. In this second series the honeymoon was over for the newlyweds, and their married life began in earnest at the Shipman family home in Essex. But, as the reality of living so far away sank in, Stacey started missing her home in Barry Island. Meanwhile, Gavin and Stacey's best friends, Smithy and Nessa, had to come to terms with their own stark reality: they're not even friends, yet they're having a baby together.
In the run-up to the 2008 Christmas special, Corden and Jones announced live on stage in Barry Island, where Jones was hosting an open-air edition of her BBC Radio Wales show, that there would be a third series of this comedy show. This arrived on the BBC's flagship channel, BBC One, in November 2009.
As Gavin starts his new job, the move to Barry Island means big changes for the whole family. Pam and Mick have to adjust to an empty nest while Gwen's got a full house again. Stacey is in her element, but will this finally be the solution to the couple's long-distance problem? And how will Gavin take to living in Wales?
The hit sitcom finished on Friday 1st January 2010... but ever since then the question of whether there will ever be a reunion is never far away.