It's just the tiniest of switches that Stewart Nicol has to flick to become Billy Connolly.
He's been living and breathing the Big Yin for years. He looks the part, and with just the slightest of tweaks to his native Fife accent, he slips into Billy's Glaswegian.
And now, after a tough few years, Stewart is ready to bounce back on the tr
ibute circuit, this time with an unusual sideline.
Having won his own battle of the bulge, Stewart is up and running with a new Slimming World group in North Bersted Community Centre on Mondays.
Inevitably, Billy takes over. At the very least, Billy creeps in. But don't worry.
Stewart is well trained in the Slimming World approach. There will be fun and laughter, but never at your expense. Stewart/Billy will make you laugh where appropriate, but there will never be a sharp side to his tongue.
"Humiliation doesn't come into it," says Bognor Regis-based Stewart. "You just don't go there. But you would hope the humour will actually motivate people."
Slimming World elsewhere boasts an Amy Winehouse tribute and a Cher tribute running groups – not in character, though the characters presumably surface.
Stewart's North Bersted group certainly isn't without some kind of precedent.
And it's certainly not without the relevant experience.
Stewart featured ten years ago on the front cover of the Observer Magazine. Inside was an interview detailing his career hopes.
He was appearing as Billy throughout the UK, mainly for corporate functions – and Stewart was convinced the big time was just round the corner.
But then things took a nose-dive. Stewart's band split up (he'd been a professional drummer for 15 years), leaving him with too much time on his hands, and he started putting on weight.
At his heaviest, his confidence suffered and he started turning down offers of work as Billy. Before the end of the year 2000, he was on anti-depressants and was heavier than ever, taking a part-time job delivering flowers to make ends meet.
And then he discovered Slimming World – or, more specifically, a local group in Bognor. Within two weeks, the first half stone had gone. Within a year, he had lost three stone and was playing drums again.
And soon there was talk of being Billy again, this time with a difference.
Instead of just appearing as a look-alike, he launched a full act.
Now he is appearing regularly – and ready to take it to the next level as Billy Connearly, complete with banjo, songs, stories and one-liners.
He's hoping to step up on the bookings. But in the meantime, he's hoping to channel his own weight-control experiences into the North Bersted group.
Helping him all the way is his wife Marian, who also runs Slimming World groups.
"It's about healthy eating, it's about attitude change," she explains.
"There are thousands of people in the UK who die because of obesity-related illnesses."
Marian is hoping the Billy Connolly connotations will make Stewart's group all the more appealing to youngsters in need of hearing the message.
"There are so many overweight children. It's bad eating. It's education – or lack of it. There is not so much exercise at school now. There is more sitting at computers."
But gradually we can all take back responsibility. Don't ditch McDonald's straight away. Reduce the thrice-weekly visits to just one. Add in some fruit and veg instead.
As Stewart says: "It's the old slimming phrase – I can't do it for you."
And who knows, if it's a Billy Connolly look-alike telling you so, you might just take note. It was certainly the way this particular Big Yin became... well, a Slightly Smaller Yin.
More details on www.billyconnearly.com or contact Stewart on 01243 861342.
