"It was literally just a fantasy, we didn't genuinely think we would get access to all this."
Built in the late 18th century and once used to give pedestrians, horses and carts access to the first Clyde Tunnel, the building has lain empty since 1980, when the tunnel closed.
The brick winding house had a brief, fresh lease of life during the 1988 Glasgow Garden Festival and then again in 1990 as the home of the Dome of Discovery, during Glasgow's International Year of Culture in 1990.
Now, for two weeks, it will be brought back to life again with theatre, music and visual arts as National Theatre of Scotland uses the venue to stage The Tin Forest Festival.
NTS associate director Graham reimagining of children's story The Tin Forest, by Helen Ward and Wayne Anderson, is a wonderful celebration of Glasgow's industrial past and its creative future.
"Like most people I've driven past the South Rotunda and thought, I'd love to do something with that. To get the opportunity is rare and I feel really privileged," he says.
"It was a challenge to get the building and it's a testament to the people at National Theatre of Scotland, we have a brilliant producing team and administrative team that negotiated and cajoled and persuaded and blackmailed probably and flirted certainly to get us in here.
"We do our festival and then it will almost immediately be turned into offices, so this is the last opportunity people are ever going to have to engage with the building."
When Graham, who is also the show's set and costume designer, started work on the project a year ago the original idea was to stage it somewhere like Glasgow Green. When practicalities, especially when the Scottish summer weather were considered, the South Rotunda seemed the perfect option.
His previous work with NTS has included In Time O' Strife, A Dolls House, A Christmas Carol and Men Should Weep, and he has also staged shows in London's West End, ancient Greek amphitheatres, national theatres and at festival around the world.
"The Tin Forest is a brilliant story about regeneration, of where you live. It's the story of an old man who lives next to nowhere, he's close to forgotten and decides to take matters into his own hands and build himself a forest made out of the detritus in which he lives.
"And out of that grows a real forest, and then he lives in the kind of place everyone wants to live.
You could see the show as an analogy and a metaphor for Glasgow and the city's regeneration," says Graham. "It also touches on the very real political and social debate that's going on now about the referendum."
He says it was crucial that a team, predominantly of Glaswegians, created the work. "This is our story," he adds.
The Tin Forest is at the heart of the Commonwealth Games Cultural Festival, an immersive puppet theatre journey that takes 10 people at a time through a labyrinth of rooms and landscapes built on the ground floor of the rotunda. Like an elaborate film set, the attention to detail is perfect and the show promises to be a truly magical experience,
"I hope people are surprised and delighted and reminded of old Glasgow and those old guys, like your grandfather, who had a shed full of dark arts: that whole generation of guys who were really skilled and diligent and hardworking. In some ways we pay a bit of a homage to them," says Graham passionately.
"It's a little bit of Glasgow's history in one of the city's historical buildings. It's incredible that in the 1970s and 1980s, when all those industries were systematically closed down, no-one took to the streets. Those guys went quietly home to their sheds.
"In a way they never really got any celebration and or recognition, that's one of things I hope this project does."
Community groups around the city are already familiar with The Tin Forest, with many involved in creating theatre events in Govan, Springburn, the East End and Southwest last month.
Govan-born actor Iain Robertson went back to his roots to work with youngsters to give classes in the Pearce Institute, offering acting, voice and communication skills with a focus on building young people's confidence. It also gave them the change to learn more about their local areas, where the ship, planes and locomotives that powered the world were built.
"There are generations of people who don't realise these things about the place where they live and the people who made the Glasgow we now know, like Mary Barbour and the rent strikes, or John McLean or Jimmy Reid," says Graham.
"These are all landmarks, without which, for better or worse, we would not be in the place we are now. It's important to weave all of that into an experience and a story that allows us a wee bit, just a moment, in the heart of when the world is coming to watch us.
"We have the Hydro over there, the Clyde and this little jewel, the Rotunda, the show will be a wee moment for people just to remember where we've come from."
l Opening Night on July 22 with Gary Lewis, Paul Riley and Barbara Rafferty in a concert in the South Rotunda courtyard.
l The Tin Forest Show, an immersive puppet theatre journey, daily from July 24 to August 3.
l Dear Glasgow, a public participatory event with a live audience, on July 31.
l International Theatre Festival, featuring 100 people from 10 youth theatre companies across the Commonwealth, from July 24-28.
l July 21-23. International Performing Company directed by Scottish Youth Theatre with 90 young Commonwealth people making street theatre at locations across the city.