The original pitch for Gavin and Stacey was VERY different to the final show

In fact, the writing duo didn’t put forward series at all, but a one-off comedy-drama called It’s My Day.

“Here’s what we started off with before Gavin and Stacey became Gavin and Stacey,” Jones, who also plays Nessa in the sitcom, told RadioTimes.com and the audience at the BBC’s Writers’ Room Festival.

“James had been to a wedding in Barry and it was an English groom and a Welsh bride. And he just said to me: 'It's amazing, isn't it? You never see anything about a wedding on TV where nothing much really happens.'"

Jones explained that after talking about the cross-national wedding, the pair ­– which were filming ITV series Fat Friends at the time – were eager to write a one-hour comedy-drama centred on one couple’s big day.

“That's all it was going to be, originally,” she said. “We did it that way and wrote a treatment. The piece was called ‘It's My Day’.

“We had lots of little vignettes and that's all it was. Just little bits of conversation at a wedding.

“We also did a backstory about how these two people met. We sent it into BBC3 and they said: 'We don't have a slot for a one-hour comedy-drama. But why don't you dramatise the backstory? You've got the whole thing there! Why don't you make the wedding episode six?'

“It went from being 'It's my Day' to being 'Gavin and Stacey'.”

Jones also revealed at the event that during the writing process, she and Smithy actor Corden act out all the characters, improvising their lines.

Explaining their method, she said: “James and I do plan loosely what's going to happen in an episode. We work out what's going to happen right at the end of the series and then what happens at the end of each episode. And then we work backwards from there.

"We work out a loose scene-by-scene – we literally have Post-its that say 'scene in garage' and that's it. Then we improvise it and take on the characters. James loves being Pam! I think it's because she reminds him of a lot of female relatives of his.”

Jones appeared on-stage alongside Uncle Bryn actor Rob Brydon, who described filming the upcoming Christmas special as “joyous”.

“I didn't think there'd be a Christmas special. And we were there and it's a brilliant script! And [I was] delighted to be doing it (even though it was too hot – it was bloody boiling during filming!).”

Revealing most of the cast were in tears in their final scenes, he added: “Everybody was often quite emotional. We laughed so much. And now I've seen it and it's rather good.

“It was the happiest job I've ever done.”

Gavin & Stacey returns to BBC One on Christmas Day at 8:30pm

BBC Cymru Wales, National Theatre Wales and BBC Writersroom Wales have recently opened their Wales Writer in Residence scheme for aspiring writers.