These old machines (most of which no longer work) are just one of the snapshots of the past that remain in the backstage areas of the Citz, which was a variety theatre from 1878 until 1945. Backdrops are still painted in the original paint-frame room, a tall, narrow space with skylight windows. Patches of myriad colour have built up on the walls after 13 decades of dripping paint.

It's not hard to see why these spaces would capture the imagination of a theatre-maker like David Leddy, the writer and director of shows including Through The Night and Susurrus, and former producer of Glasgay!, who is known for his site-specific work.

"I had a meeting with Jeremy (Raison, the artistic director of the Citz] and he said very cryptically, 'I want to show you some spaces,'" Leddy says. "Walking round, he said, very casually, 'Wouldn't it be great if someone did a site-specific show in all these spaces?' He knew I wouldn't be able to resist it. Within a week, we'd done a funding application, and a week after that I began writing."

The result is Sub Rosa, a gothic drama about the untimely death of a chorus girl, which sparks a "red velvet revolution" in a Victorian variety theatre. It is an epic promenade through the unseen spaces of the Citz, from the wardrobe department, with its bulging rails of dresses and cupboards of wigs, to the disused upper circle of the auditorium.

"Twenty-six different areas – as I keep being reminded by the production manager!" says Leddy. "It's a huge job technically. In wardrobe, there's one plug in the whole room. You end up spending your budget on cable. And, paradoxically, for a show about someone burning to death under the stage, health and safety has been…" he pauses, looking for a diplomatic phrase… "very complicated."

Leddy's "plucky gothic heroine" is Flora McIvor, a chorus girl who won't give up when her best friend – and the other half of the Siamese Twin Princesses act – burns to death one night while waiting for her cue. Her determination to bring the theatre's managers to justice leads her into a world rife with secrecy, pretence and deceit (sub rosa is the legal term for secrecy). "Along the way, we meet other performers: Vesta della Verita, the Italian light operetta singer, who's really from Kilmarnock and is the best pickpocket the Barras has ever seen, and Vaclav the valedictorian, serpentine strongman, contortionist and posturer.

"I'm fascinated by the Victorian era because we think of it as very stuffy and buttoned-up, but it was a period of massive social change, the dawning of modernity. Also I'm interested in the brutality of the workplace and people dying, particularly in theatres. People would regularly burn to death under the stage while the show carried on above their heads because they were wearing flimsy little fairy costumes and everything was lit by candles." Leddy wrote the bulk of the play during a residency at the Global Arts Village in Dehli. "At first it was really strange to be writing about Victorian Glasgow in contemporary Dehli, but within days I started to see such strong parallels. That sense of people being easily disposable, there being nothing like employment law, nothing like health and safety. You regularly see women riding side-saddle on the back of motorbikes in flapping saris at 50mph, carrying two babies in their arms. It was terrifying."

Of all Glasgow theatres, the Citizens' perhaps has the most palpable sense of the past, in the tawdry glamour of its red-and-gold interior and the ostentatious elephant statues in the foyer (salvaged from the Palace Theatre next door, which was demolished in 1977). Refurbishments, adding new sections on to the old, make it more of a maze than ever. "It's a labyrinth back there," says Leddy. "You feel lost very fast. I have a very good sense of direction but it took me a long time walking around the building to know my way about. It's just a rabbit warren."

Leddy says he made it clear from the beginning that he would write a fictional story inspired by the spaces, rather than drawing on the history of the building, rich though that is. "In the first production in 1878, there were elephants walking on travelators on the main stage in a production of Ben Hur, which is incredible," he says.

The theatre warns that the show is "not for the squeamish", drawing on the tradition of gothic horror. Much of the artifice is about letting already evocative spaces do their work, with the help of acclaimed lighting designer Nich Smith and sound-effects ranging from the scratching of death-watch beetles to the cracking of 10,000-year-old ice.

Sub Rosa is Leddy's biggest work to date, featuring a stellar cast including Cora Bissett, Alison Peebles and Finlay Welsh. His last show, Home Hindrance, was performed for an audience of six in his Glasgow flat, using actors and fictional characters to explore the experience of living with someone with a serious illness (his partner, Calum, is a dialysis patient awaiting a kidney transplant).

"It was a very emotionally difficult show because it dealt with my partner having been chronically sick and nearly dying, and we relived that night after night. You're like, 'Why did I think this would be a good idea? This is grim!' But I was very pleased with it as a show.

"I think in many ways it is similar to Sub Rosa. They're both tours around a particular space, about that space, they're sets of monologues which describe events in which the major figures are absent. In terms of atmosphere, they're completely different, but I feel like this is a natural progression."

Although he is also an acclaimed performer, with a triptych of solo shows under his belt, Leddy says he now intends to concentrate on writing and directing. His next project is White Tea, a collaboration with Japanese-Canadian company Theatre Replacement, based in Vancouver, which will be performed at the Tron.

"I always aim to make things that are quite different. I try not to do the same thing again and again. I always admire people like Ang Lee or Steven Soderbergh, people who don't really have a style, who are able to have a style which fits the story that needs to be told."

15th January 2009