The actor formerly (and still) known as Barry from EastEnders, not only loves strolling around supermarkets, if he sees a jam jar that's a little askew, he'll move it a half an inch.
A bottle of brown sauce in the wrong place? He'll shift it.
"I do this all the time," he says, grinning. "Although I wouldn't want to go through the whole shop, moving stuff in case I get reported to authorities."
Shaun's minor obsession with supermarket stacking stems from the fact he was once set to begin a shop career.
"I worked at Safeways on the night crew," says the 50-year-old who grew up on a Kent council estate.
"Then the manager twigged I had half a brain and said to me 'Why don't you come on our management training scheme?'
"But at this time I had got the acting bug and said 'I'm going to try and get into acting school!' And he suppressed a giggle. But I got in to drama college and never looked back."
Shaun adds, grinning: "But I still enjoy walking around supermarkets. They're very interesting and peaceful, places. Shaun Williamson has to be one of the most grounded, self-deprecating actors in the business. The impression he gives is he's utterly thankful for being able to be a performer.
He's certainly delighted to be one of the hugely talented cast of the major hit comedy One Man, Two Guvnors, the madcap story of Brighton gangsters, skiffle bands and women posing as dead brothers.
"I just love it," he says of his role as Charlie 'the Duck' Clinch. "The adrenalin you need to appear in this show is amazing because if you don't make your entrance on time the ball drops. And it's wonderful to be part of this cast."
Shaun didn't get into acting until he was 27 when he joined an am-dram group. But all the signs were he was set for a career as an entertainer.
"I've always liked people, so I did a variety of jobs when I was young."
Indeed. He spent three years as a postman, (getting the job, he says, largely because his dad was a postman) joined the navy; dropped out of the navy, and spent his 20s 'drinking and drifting in and out of temporary jobs.'
"I was a Pontin's Blue Coat for a while and an 18-30 Club Rep," he recalls. He also spent time in US summer camps, where rich parents pack their kids off for the summer, leaving young Brits to entertain and instil sensibility in the youngster.
"I worked in a camp in the Catskills where a lot of the kids had problems. A lot of them were on Ritalin. Or they should have been.
"They'd pretend to take it and go mental and race off into the woods and we'd have to get the state troopers out because of the danger from bears."
Shaun grins as he recalls his own bear problem, which came about while he was 'entertaining' a young lady.
"Oh, there was all sorts of things going on that summer," he recalls, a nostalgic smile beaming on his face. "But then one night I was in the woods with a girl and we got disturbed by a bear. That was a scary.
"But overall, I loved working with the kids. It was very rewarding."
On leaving drama college, Shaun had some small roles in television before landing the role of tragi-comic Barry. On leaving ("fed up playing the village idiot") he joined Ricky Gervais's Extras, in which he lampooned himself.
Shaun's willingness to be the butt of the jokes was, at the time, revelatory. He was prepared to have his hair transplant referenced. He was prepared to answer to the name of Barry.
But then there's a real humility about the man, who when he's not working in TV or stage musicals such as Guys and Dolls, does after dinner speaking or cabaret.
"I don't have a career plan or anything. And I'll do whatever, so long as it's not too demeaning." He adds, laughing: "I've got an answer phone message which says 'I'll do it!'
"I've got one speech for the after dinner speaking, a false and crazy autobiographical story of my life, and believe it or not I've actually done the Loo of the Year Award, the Food Packaging Industry Awards.
"It's the same speech, but what I do is I find out a bit about the company, find out if the MD is always on the golf course, who's got the Ferrari, and take the mickey out of them."
He added: "I've also got the cabaret act I take around the holiday camps, eight songs with five minutes of gags in between. "I can charm a Pontins or a Haven Holiday camp audience sure, but if I went to the Comedy Store and began with 'I'm not saying my mother in law's fat . . . but,' and then tried to sing Sweet Caroline - I wouldn't last two seconds.
"That's basically my act."
Yet, he's certainly not constrained to Pontins.
"A couple of times a year the phone goes with a very nice job," he says of the likes of One Man Two Guvnors."
But what of the bear in the woods story, Shaun. I take it you managed to escape with your dignity intact?
"Yes, but the girl I was with got lime disease from the woods," he says, shaking his head.
n One Man, Two Guvnors, the King's Theatre, until Saturday.