There's always a buzz and a crackle about the writer/actor/producer, which you'd expect from a co-founder of zeitgeist theatre companies such as 7:84 and Wildcat. There's always been a tangible excitement as he talks about future plans.
Today, MacLennan's costume is still bright and theatrical; he's wearing sunburst-coloured cords, a fire engine red body warmer and the ever-present 1950s Tour de France moustache. However, the voltage is a little lower as we meet, ostensibly, to talk about the new Play, Pie and a Pint (PP&P) theatre series.
For reasons to be explained, the chat is softer, calmer. And there's an implicit understanding that details of the new plays will remain in the margins of our conversation. What we need to talk about is his career. What we need to gain is an understanding of what has made MacLennan the most important figure in Scottish theatre in the past 40 years.
We begin with the astonishing success that is Oran Mor's PP&P series. Lunchtime theatre wasn't a new concept; it existed in London and in Bewley's in Dublin before coming to a converted church in Glasgow's West End. But MacLennan added pies and pints - and some great plays - to the menu, and in doing so arguably changed the face of world theatre.
"I'm not sure that's indeed the case," he says, grinning, his hyperbole antennae twitching a little at the suggestion. "But I like to think we're making a difference."
MacLennan's modest reply doesn't do the concept justice. Not only have audiences risen year on year, the success story is also being replicated in Edinburgh, Ayr and Aberdeen. Co-productions operate throughout the UK and the winning formula has been franchised out to Philadelphia and soon Caracas and Sao Paulo. Oran Mor writers' efforts have been translated into Italian and Russian, performed in New York, Moscow, Adelaide and, soon, Paris. And thanks to MacLennan and co, bite-sized chunks of Scottish culture are being served up with pies (or pizzas in Philly) around the planet.
But here's why the talk is not about detailing future plans to open in Venezuela or wherever. The dynamo that is David MacLennan is slowing down. The 64-year-old was diagnosed with Motor Neurone Disease a year ago.
And yet the positivity which saw him once troop his troupe around the Highlands and Islands in rickety vans playing to tiny audiences - and sometimes roaming animals - hasn't left him. The fun in the man, highlighted by one Oran Mor play appearance wearing nothing but a giant nappy and a pleased expression, is far from gone. "I've been remarkably lucky," he says of his life. "I think one of the things about getting this wretched illness is it focuses the mind on how lucky I've been with my work, my family and my friends. Until I got this I wasn't quite as aware as I should have been. Then when you get what my neurologist called 'a catastrophic diagnoses' - he wasn't mincing his words - you've got to ask 'Will this define me?' At some point I guess it will, but for the moment, I'm going to carry on."
The producer admits to feeling "a little fatigued" and chat is broken while he swallows a bitter vitamin cocktail. But he explains why he won't slow up on theatre work.
"I've loved it since I was around six-years-old and my parents took me to the King's Theatre in Glasgow to see Peter Pan," he recalls, his eyes shimmering. "As the little Tinkerbell light faded, Peter Pan came forward and called out; 'Boys and girls, Tinkerbell is dying! But we can save her! I want you all to stand up and shout out 'I believe in fairies!' Before I knew it, I was standing on top of my seat shouting at the top of my voice. And I've been captivated by the magic ever since."
Over the years, MacLennan's doctor parents, (his father was the eminent obstetrician Sir Hector MacLennan) drip-fed their son with a range of theatre, from high drama to variety. "Jimmy Logan moved in next door and we'd get invited, en famille, to the Alhambra Theatre. I'd visit Jimmy in his dressing room, with all the wonderful mirrors, costumes and make-up boxes then go backstage and see the Bluebell Girls dancers close up, with their legs all the way up to their oxters." His moustache curls as he grins. "It was lovely."
The boy David made his stage debut at boarding school in Charley's Aunt, playing Lord Fancourt Babberley - in drag. He'd fallen in love with acting. "I don't think my parents fell in love with the idea, though.They would have been happier with a stethoscope around my neck as opposed to a string of beads. But unfortunately, as an actor I was second division." Was it an inability to surrender self? "Could be that," he muses. "But it could be I've always felt myself outside it, as writers do. I sort of looked in on a production."
But how to harness that overview? The lightbulb moment came with a trip to London to meet up with his actress sister Elizabeth, her boyfriend John McGrath and seeing Joan Littlewood's conflict indictment Oh What A Lovely War. "It blew my mind. I thought, 'This is exactly what I'd love to do.'"
In 1971 MacLennan co-founded 7:84 Theatre Company with McGrath. Ibsen, Chekhov (and often the Fourth Wall) were ignored and theatre in Scotland had its own voice. In 1978, the voice sang louder when he formed Wildcat with Dave Anderson, again mixing social message with melody. "Wildcat lasted 20 years," he recalls. "We never got rich, but we never stopped turning out plays. I'm so lucky I came into the business when it was growing and helped fertilize the soil. And it was fun, sharing attitudes and politics."
And often the back of a van? "Oh, yes," he says, "and one night in Bo'ness village hall our play was plunged into darkness while we found 50p for the meter." MacLennan's theatre, however ,wasn't all about selling dialectics with the half-time ice creams. "One of my best moments was watching 1600 people at the Pavilion in Glasgow standing on top of their seats singing 'Champion-es!' at the end of our Celtic Story."
It was serendipitous when MacLennan met Oran Mor boss Colin Beattie. But in truth, MacLennan (voted The Stage's Producer of the Year in 2013) was the only man who could make the PP&P idea work. The concept needed a businessman whose business was the theatre. It needed someone who could understand scripts and writers, find and grow acting talent - and call upon the likes of Elaine C Smith and Robbie Coltrane to perform dinner-time theatre for less than a dinner lady's wages.
It needed someone who could say "no" to hopeful writers, but with a grace that leaves them feeling lifted, a grace reflected in MacLennan's customary audience address: "If you have a mobile phone, now would be a fantastic time to turn it off."
That grace re-emerges when he praises his supporting cast; Beattie, of course, his chum Anderson and his production team. And Glasgow. "No other city, with its insatiable appetite for entertainment and broad mindedness, could have played home to the concept," he says.
Yet, Glasgow needed MacLennan's vision, energy and love for theatre. "Today, people live in a virtual world, the world of the screen," he offers. "There are so few ways of coming together. But theatre is the opposite of that. I'll never tire of seeing a group sharing a space, a table, food, comment - and laughter."
It's no real surprise MacLennan is married to actress Juliet Cadzow. Their careers criss-crossed over the years until the pair became a permanent double act at Elaine C Smith's wedding in 1988. "Juliet kindly invited me back for coffee - and I never left." And there's no doubt MacLennan the scriptwriter would have come up with a different ending to the final act.
"I can imagine choosing to die in the middle of the Tay with a salmon on the line and a hip flask in my pocket, going out to a giant heart attack at the age of 104," he says with a wry smile. "But it's not to be. And you know, I've had a quite fantastic life."
And with that he's striding home for an appointment with his laptop, to write his memoirs and come up with cracking lines and agitprop gags that will crease a summer panto audience. He's still smiling. Still wearing bright costumes. And he still believes in fairies.