Ruth, who co-created the comedy with James Corden and plays Nessa, enlisted the help of a family member in a bid to keep the reunion under wraps.
Ruth has revealed that on a reconnaissance trip to the Welsh town of Barry, where the show is set, she took her nine-year-old niece along with her to use as a cover story in case she was spotted by fans.
The actor was there to speak to Glenda – who owns the house used by Stacey’s mother Gwen in the show – about using the property again for the Christmas special.
Ruth admitted she felt “ashamed” at using her niece but said it was necessary to keep the show’s return a secret.
She told PA: “When I’ve been back to Barry sometimes there are people who are taking photos of the outside of the house and, if they see me, they sometimes want to take photos. If they see me on my own, it might start suspicions going.
“So I took my nine-year-old niece, who called the show ‘Gayin and Stacey’, she doesn’t know anything about it – ‘Let’s show you where Auntie Ruth used to film a television programme’.
“So, I took her and had some photos taken outside so I could just say ‘I’m just showing my niece’.”
Ruth also had to tell a white lie to her friends, who asked about her frequent trips to Los Angeles to write the show with James Corden.
James, whose busy schedule hosting The Late Late Show complicated matters further, would squeeze in writing on his days off.
So Ruth flew to California in September last year before returning in February, arousing the suspicions of her friends.
“My friends were saying to me ‘You’re going out to LA again?’ and I went ‘Yeah’ and they went ‘Is it for work?’ and I went ‘No, no just going out for a little holiday’. ‘Oh, you like it out there, don’t you?’ ‘Yeah, yeah, it’s nice’, she explained.
“The truth was we weren’t lapping up the LA sunshine, we were stuck indoors … not stuck, we were indoors writing.”
Gavin And Stacey stars Mathew Horne and Joanna Page as a young couple who fall in love following a whirlwind romance.
The show features a strong supporting line-up of memorable characters, including Rob Brydon’s eccentric Uncle Bryn and Alison Steadman’s over-protective mother, Pam.
Larry Lamb and Melanie Walters also star.
Gavin & Stacey returns to BBC One on Christmas Day.