The Late Late show host, 41, and writer, 53, said their first draft for their award-winning series just “wasn’t good enough”.
Speaking about the process to the Radio Times, Corden said: “We’d written about 40 pages, and we sat and we read it.
“And we both went, ‘This just isn’t good enough. It doesn’t feel like the show. It doesn’t feel like it’s right.’
“There was a real silence in this room, wasn’t there? A sense of… maybe we were naive to think we could pick it up.”
Jones, who flew over to Corden’s house in Los Angeles for the writing session, then took a break from the writing while Corden said he “moped around” about the episode.
Later on, the duo then identified the issue was the draft did not have “a heart to it – a big moment that resonated”.
Corden added: “We went round and round, and it was probably about midnight – Ruth here with her laptop, me pacing around – and as soon as we had let it go, an idea arrived, maybe the quickest scene we’ve ever written, really.
“And as soon as that scene was finished, we had no choice. We said, ‘Well, now we have to make it.’”
Corden and Jones announced in the summer that they were writing a new episode nine years after the series three finale.
The show documented the romance and marriage between a couple, Gavin Shipman and Stacey West, who met online, and the union of their families.
The Christmas special, which will air on BBC One on Christmas Day, sees our favourite characters catch up over a Christmas dinner at Bryn West’s (played by Rob Brydon) house in Barry Island, Wales.
On Saturday, Jones, who plays Stacey’s best friend Nessa in the show, turned on the Christmas lights for Barry Island – the town where Stacey’s family and friends are from.
According to the BBC, she addressed the crowd in character, shouting some of her well-known lines such as: “Ow” and “What’s occurin'”.
She added: “Cracking to see you all tonight… have you got nothing better to do?
“Oh, there’s some lookers out here tonight, a few exes in the crowd here I can see.”
Read the full interview in the Radio Times magazine, out now.