You Me Bum Bum Train: The latest journey into challenging immersive theatre

As a volunteer at the immersive theatre production of You Me Bum Bum Train, I've been able to do things I wouldn't normally do. I've fixed two sewing machines, I've lugged furniture around, I've painted walls and I've felt incredibly capable and resourceful while doing them. Tackling things outside your comfort zone is at the heart of the You Me Bum Bum Train experience, where an audience member, or "Passenger", is thrown into the heart of the action.

From tonight, Passengers will arrive at the old Foyles bookshop building in London where the new YMBBT show takes place, and be hurtled from one short scene to the next, in each of which they have to improvise their part while the rest of the cast react. The Passenger has no idea what is going on behind each door and the YMBBT team would like to keep it that way. They don't even have publicity photos. Instead, the founders strike silly poses against surreal backdrops – see right. So I can't reveal what's happening this year. But previous scenes have involved discovering you're the head of MI5 and making a world-changing decision or having to operate a forklift truck without any guidance.

In each scene the audience member is the focus of attention and the cast of volunteers – who aren't professional actors but who often have skills or experience relating to the context of the scene – interact with that Passenger. Each scene is timed and during the one I was cast in we had about two minutes before resetting and then running the scene again with the next Passenger. There are about 70 Passengers passing through in one night, so it's frantic.

Kate Bond and Morgan Lloyd founded You Me Bum Bum Train at art school in Brighton in 2004. It was held in the basement of an office block. "I found it very depressing trying to find something that meant something to me at art school," says Bond. "A lot of art is very egocentric but what I love about this is there is no one leader and it's not a production where every scene is rigidly fixed, so it's accessible for everyone. No volunteer ever gets turned away."

YMBBT has grown to huge proportions. It was awarded the Oxford Samuel Beckett Theatre Trust prize for its show in 2012 at the Barbican in London and an Olivier award for outstanding achievement. Stephen Fry, Dominic West, Jude Law and Sir Ian McKellen are just some of the show's celebrity fans, but there aren't many detailed reviews or articles about the experience. That's because secrecy is key.

"If a Passenger has been forewarned then they always say they regret knowing about it," says Bond. "In the early days, people would just find a flyer in a pub saying You Me Bum Bum Train, a time and a location and nothing else."

In a recent show, one Passenger had been told by his friend that they were going to see Billy Elliot and had no idea what would happen. "He had to take a break from the show because he was shaking and he just wasn't prepared for what was going on, but he said it was amazing, he just felt overwhelmed."

"A lot of the shy people say if they knew what they were going to do they would never have taken part but they get a huge confidence boost from realising they can."

The show is run on a shoestring budget; props are scavenged from websites like Freecycle and car boot sales. It's amazing how detailed and realistic they are considering they started with a building site three months ago. In one of the scenes I rehearsed for, the scene director suddenly stopped talking to examine the ceiling. "It still needs cornicing. It won't look right without it," he said. The cornicing was added the next day.

YMBBT receives a grant from the Arts Council to help with running costs, and Bond and Morgan pay themselves a small wage (Bond is on working tax credits), but the army of volunteers are all unpaid, aside from being given meals. "It would be nice if Bum Bum could give back more," says Bond. "We have a fantasy of treat chutes going through to every floor with snacks and vending machines and making it more Willy Wonka for all the volunteers, but we haven't been able to yet."

They've been criticised for not paying, but the production couldn't happen any other way, Lloyd and Bond worked out that a ticket (£48.50 for this production) would cost around £2,000 if they paid their volunteers minimum wage and broke even on the running costs.

The best bit about the volunteer experience is that people from all walks of life and all ages get involved. "It makes people more open-minded because it is such an open-door policy and you meet people from different backgrounds," says Bond. "We had a lawyer who asked to volunteer and afterwards she became a human rights lawyer instead of a commercial lawyer because of the experience."

The bonding element has even produced some Bum Bum marriages over the years, says Bond. "A bit like going to war, it brings people together, and they achieve things that are really huge."

'YMBBT' runs until 30 October at the old Foyles building, 111 Charing Cross Road, London. Tickets are sold out. But to volunteer, go to bumbumtrain.com/get-involved