On the surface everything seems calm and controlled but underneath he's paddling like fury.
He doesn't like having his photo taken and, having just good-humouredly endured an early-morning photo call, in which he discovered 97 ways to pose with a stage curtain, he's ready for a cigarette and coffee to kick-start his heart.
It's over 20 years since he starred in Blake's 7, and now the 54-year-old Galashiels-based Welshman looks more like a favourite uncle than an intergalactic freedom fighter.
"Most people will know me from television," he admits, "after all, one episode of Blake's 7 will have been seen by more people than all the Royal Shakespeare Company shows I've done put together."
And despite numerous series since, and with Shakespeare bringing him to Musselburgh later this month, it's still Blake that follows him around.
"People stop me in the street and say, "Oi, you know who you used to be don't you?'," he guffaws affectionately, a bellow of a laugh, as he continues, "I always answer, "yes and I still am'." It's this approachability, and down-to-earth outlook that sets him apart from many in his trade.
His ethos is simple: "My aim is to spend my life trying to make people happy." As Blake he did just that, but two years in, and with the series at the height of its popularity, he jumped ship.
Sidestepping the typecasting curse brought with it difficulties of its own. "The problem with avoiding being typecast is that as time goes on you are never first choice for anything.
"Slowly though, I'm beginning to get good varied parts," he confides with a heartfelt wish that maybe one day he'll get to play Falstaff. "I love Shakespeare. The writing is just phenomenal, especially when you think that he was writing 400 years ago and is still teaching us ."
And it's this love of Shakespeare that brings him to Musselburgh's Brunton Theatre.
Playing Sir Toby Belch in their production of Twelfth Night, his character is normally perceived as a jolly japester - but Thomas has his own ideas.
"I'm trying to bring out the dark side of him. A lot of drunks who are jolly and cheerful can, at the flip of a coin, become very nasty, and then flip back again. It's that aspect I'm trying to capture."
He has two-and-a-bit weeks to find that essence , but there's one great role he regrets he will never play. "I'm about the right age now to play Othello, but I'll never get the chance as it would be politically incorrect now for me to do so."
In recent years he has moved to Scotland, a bad career move at first. "London thought I'd retired or died," he says. "And Scotland didn't know I was living here. So things took a nasty nosedive ."
Choosing Scotland as home was not difficult, and by moving to the Borders he rekindled a love affair with the country and Edinburgh that began when he was 13.
"My parents moved to Edinburgh in 1958 and lived there for four years. I was at school in Canterbury and came up in the holidays. I remember travelling by steam train, and that if you flew it was £9 return standby."
The family resided at 26 Cluny Gardens in Morningside, much to the delight of schoolmates. "All my friends used to write to me at 26 C loony Gardens," he remembers with glee.
His visits to Edinburgh continued. As a student he hitchhiked up from Newcastle, and like all true Welshmen he could regularly be found at Murrayfield for the rugby.
As a raconteur he is at ease telling tales and painting hilarious scenarios.
He came to acting almost by default. "I got into RADA, mainly because I wanted to continue being a student," he admits. Halfway through he realised he'd stumbled upon his true vocation, although his first experience of stage and screen was not uneventful.
"In my first theatre production my big scene involved walking on stage and opening a door for somebody.
"I walked on to the stage, opened the door . . . and it came off its hinges. So at the end of my first ever professional stage appearance I had to pick up the door and walk off stage with it."
The audience loved it, as they did the time he lost his tail as King Rat in panto by trapping it in a door, and you'd be forgiven for thinking he is accident-prone. "No I'm not really," he quips, "but I remember one of my first jobs after drama school was the film Quatermass and the Pit."
He was the man in the classic British horror who discovered the alien skeletons in the London underground. "They built up this very expensive plaster of Paris tube station wall with real clay carefully put in and the alien skeleton set behind it.
"The director told me to take a pickaxe and hit the top of the clay so that the whole section of wall would fall away. He suggested a rehearsal first, and warned me not to actually hit the thing.
"So I swung the pick, stopped it dead an inch from the wall ... and the head flew off and smashed the whole thing. "There was a moment's absolute silence broken only by the director yelling "props'! It took three hours to rebuild."
Hopefully no such mishaps will befall him when he takes to the stage as Sir Toby Belch on his nerve-wracking first night.
Gareth Thomas stars in Twelfth Night, Brunton Theatre, Musselburgh, Jan 28 - Feb 12. Call 0131-665 2240 for tickets.
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Last updated on 29th of January 2000.