In Tellyland, it’s called "jumping the shark". It’s the moment when the stars of a successful show fall in love with themselves, think they’re invincible and promptly lose the plot.
For Ford Kiernan and Greg Hemphill, it threatened to come when Bobby Carlyle phoned up to ask: "Could I be in Chewin’ The Fat?"
"All good shows are eventually tempted to jump the shark," explains Hemphill, the tall one with the cultured Canadian accent in the BBC Scotland comedy double-act. "Aye, it’s when the folk in it get right up their own arses," adds his bauchly buddy Kiernan.
The jumping-the-shark moment takes its name from Happy Days. After years of success the American sitcom’s regulars left the safety of the soda parlour and headed down to the beach, laughing and backslapping as they went.
Then, fatally, The Fonz swapped his bike for a jet ski. Things were never the same after that.
"There’s a website where fans post the episode where their favourite show died," continues Hemphill. "In Roseanne, for instance, it was when she won the Lottery."
It’s usually always terminal when a show tries special effects, or goes to an exotic location. Witness Cold Feet only this week: the comedy-drama couldn’t resist splashing the proceeds of its big-spending viewer demographic on a show-offy Sydney finale which somehow managed to squeeze the Harbour Bridge into every shot.
Another warning sign is the guest spot ... so what’s with Carlyle’s two-minute turn in Chewin’ The Fat’s Hogmanay special? "I know, it looks bad, but Bobby’s a fan of the show," says Kiernan. "He wanted to do a Bish & Bosh sketch because he used to be a painter-and-decorator, but we decided to make up this routine about the filming of a First World War movie called Trench Of Fear and have my crap-actor character Ronald Villiers take the pish oot o’ him."
There’s more: Hootsy Coochie is another film spoof, an Elvis-comes-to-Skatland affair, complete with seven brides and seven brothers stepping gaily in Irn-Bru wigs - surely such a big musical number plunges them even deeper into shark-infested waters?
"Well, we’ve got a network budget now," adds Kiernan, "and if you’ve got the money, it’s a shame not to spend it. We’ve always been over the top, anyway. In the last series we blew up a garage. Have we jumped the shark? Dunno, but if so, we’ll just turn round and jump the f****r all over again."
The pair are looking pretty prosperous themselves right now. Smart suits, subtle shades of black and grey, and Hemphill’s even wearing a tie. It’s "The Rudest Men In Scotland", dressed by GQ magazine. But appearances can be deceptive, and Kiernan is soon picking the raisins out of the BBC Scotland scones and spilling his coffee everywhere - he’s a right slaister.
The Hogmanay heuch is a taster for fourth series of Chewin’ The Fat starting next month, which will be the first to be screened simultaneously in England. Some watering-down might have been anticipated, but the duo are anxious to stress that the show’s heady mix of Buckie, Carly Special and Sunny D remains unaltered from the original formula which turned them into such a couple of cults.
"We’re delighted we’re being shown down south now, but this is first and foremost a programme for Scots," says Kiernan. So, no subtitles for "big hairy walloper" then. Try and guess what one of them is.
The new series will feature some new faces: neds who travel back in time to pioneering, prospecting America, Big Jock, a golf-club bore, a superhero called Captain Nippletwister - and a team of farting gymnasts. To make room for them, old favourites have had to be killed off, among them the lighthouse-keepers of Aonoch Mhor. They’re not gonnae dae that one anymore.
George goes, too. Alongside the champion boozers, chain-smokers and deep-fried death-wishers, this bullying husband represented the dark side of Chewin’ The Fat.
Have the pair gone soft? "No way," says Hemphill, "a lot of the new stuff is pretty challenging." And he flashes a menacing grin with his Children Of The Damned eyes.
"We’re always asked where the dark stuff comes from," adds Kiernan, " Neither of us had a terrible childhood. But we’re from Glasgow, the humour is black here, and it’s the same in any river town. There’s a lot of comedy in poverty and folk trying to dig themselves out o’ holes."
Next year will also see the first Chewin’ The Fat spin-off: a sitcom featuring old-timers Victor and Jack called Still Game.
"We started it two days after we finished the new series, so that’s a whole year we’ve spent staring at each other’s ugly mug trying to raise a laugh," says Kiernan. "Christ, our sons were born ten days apart and it’s almost telepathic between us now. We’re starting to get on each other’s nerves because we’re aye finishing each other’s … "
"Sentences," says Hemphill, with perfect comic timing.
Chewin’ The Fat Hogmanay special is at 11.15pm on BBC1 on 31 December. The new series starts on 18 January For TV disasters, see: http://www.jumpingtheshark.com/