The much-loved BBC comedy is the highlight of the BBC’s festive offerings this year, almost ten years after the final episode aired.
Speaking to Red magazine, Ruth said that working with her co-star and co-creator again “was a bit strange initially”.
“Although we’d seen each other over the years, we hadn’t spent any great length of time together,” she says.
“But what’s nice about the way we write is that we just act out the characters ourselves, so it’s just like playing, really. We make each other laugh a lot.
“Ever since it finished in 2010, we’ve always said, ‘Do you think we’ll do another one?’ Sometimes we would just text each other and go, ‘I have a really good idea for Bryn’.”
The special will also see Joanna Page return as Stacey alongside the other supporting cast members – Rob Brydon, Larry Lamb, Melanie Walters and Alison Steadman.
Ruth says that she couldn’t rule out the show coming back again, but admits there aren’t currently any plans to do so.
“We will never again have the effect that we’ve had this time around, having said for 10 years that we’re not doing any more and then announcing something special,” she says.
“I think it would be churlish of me to say there won’t be any more. The honest truth is there are no plans, but I don’t think I could ever say never.”
The show told the story of Gavin, from Essex, and Stacey, from Barry in Wales, who fell in love and married after speaking on the phone to each other every day at work.
It aired over three series, plus a Christmas special, from 2007 to 2010 on the BBC.
Last month, James - who plays Smithy in the BBC comedy - admitted that when he sat down to write the Christmas special with Ruth, they got 40 pages in before realising that what they had written wasn’t up to scratch.
James said the pair took some time out from the writing process in a bid to save the project.
The January issue of Red is on sale from December 4.