Jail Caesar! A new film about the Roman leader features a supporting cast of prisoners

It starts scatologically. It starts, to be strictly accurate, with a man reading a poem about a prison cell, and then with a man in another prison cell talking about how he used to "push a cartload of shit". It wasn't, he says, "a bad job". The "screws", he says, "left you alone". And one day, he says, "a young guy comes up to me, a civvie, unescorted, and says 'I want to make a film about the early life of Julius Caesar'."

The man's a "lifer", a real "lifer", in a real prison. He's telling the story of the film he's in. The screen splits and suddenly there are more prisoners, in different uniforms, in different prisons. There's a voice telling us that "string theory leads to the possibility of multiple parallel universes". There are statues of Julius Caesar, in stockings and lipstick, and graphics, which look like graffiti, saying things like "dictator", "bisexual" and "laughing stock". And then we're into the action: a young man in an orange jumpsuit in a crowded prison cell, sitting on a "shitter".

The young man is Julius Caesar. The prison he's in is Rome. The prison he's in is actually Pollsmoor, in South Africa, where Mandela spent six years of his life, and where gangs rule. "If a guy crosses another guy's territory," says the real head of security in an interview on the website for StringCaesar, "they'll kill him". And in this film, which has just been nominated as best UK feature of the Raindance Film Festival, they nearly do.

Julius Caesar is played by Warren Adler. His uncle Marius is played by John Kani, and Marius's rival leader, Sulla, is played by Derek Jacobi, on mesmerising form. "Kill who you wish to kill," says Jacobi's Sulla. "Rape who you want to rape," says Kani's Marius. "Burn," says Jacobi's Sulla, "wherever you want to burn". In the hour and a half that follows, the men who follow them do. This is power: the brute struggle for power that has run through every society in every world. In this film, it's the struggle between Sulla and Marius, and the struggle between Caesar and Sulla, and the struggle between Caesar and anyone who holds him back. Jacobi is chilling as a vicious dictator, and so's Kani and so's Adler. But what's even more chilling is to know that most of the people in the film – the 90 per cent of the cast, in fact, who are real prisoners – live like this all the time.

As you see Adler's Caesar trying to seduce a big, fat gang leader of another prison, and then the "pirates" who have kidnapped him on his way to somewhere else, and as you see the way people look at him, with fear, and envy, and awe, you see how brute power has always worked: with any tools, and at any price. If the detail of the plot isn't always easy to follow, it's more than made up for by the energy and, perhaps surprisingly, the poetry. Many of the actors may not have had much in the way of formal education, but they speak the lines, from the very poetic script, as if they wrote them. One or two even did. The poem that opens StringCaesar was written by a prisoner, and so was a rap about Caesar, delivered in a crowded "court". This is certainly a film about Julius Caesar, but it's much, much more.

The filming, in Pollsmoor Prison, and Cardiff prison, and also, before the governor moved on for one who wouldn't allow it, in Brotherhood Lodge Penitentiary in Alberta, took just a few months. The work that led up to it took years. It started, in fact, when Paul Schoolman, an independent director and film-maker who studied acting with Jacques Lecoq in Paris, picked up a history book about Julius Caesar. "It said he was a failure until he was 33," he tells me, "and was a laughing stock and gay. Then one day he was ruler of the world." Schoolman went to the British Library to find out more, wrote a "promenade piece" which got rave reviews at the Edinburgh Festival, and then got the idea of doing it in prison. The Home Office said he could go to Dartmoor, and do workshops, which, with his wife, the actor and producer Alice Krige, he did. But it took more than 20 years before they were finally able to get the project off the ground. When they did, it was mostly in South Africa, where Krige grew up.

"It's an experience that generates an enormous amount of emotion," she tells me, after showing me the film in the sitting room of the mews house in Hampstead that she and Schoolman inherited from a friend when their home in California burnt down. "When you're working with prisoners, everyone's working on a voluntary basis, and nobody has said to them before that they've got anything of value to give. So," she says,"they give without reservation, and that's an extraordinarily moving thing to be part of."

Krige and Schoolman met on the set of Chariots of Fire (where she played the fiancée of Harold Abrahams), but Schoolman seems uncomfortable when the subject comes up. "It's as if you're trying to hitch a ride," he says, "and I'm not. I haven't even got it on my CV. My background was as poor and dysfunctional as the people I work with."

His passion, and Krige's passion, for the project shines through. It was their passion that persuaded Derek Jacobi, and his partner Richard Clifford, to put their names to it years before it was filmed. It was their passion that persuaded prison governors to suspend all normal rules. And it was their passion that persuaded prisoners to take part, and to do it as if what they were giving was their soul.

"This movie," says Denwar Ohlson, the head of security at Pollsmoor who appears at the end of the film, "has changed so many people's lives. Nobody that I know of who came out of prison after the filming is back." It has changed the lives of the professional actors too. Derek Jacobi burst into tears when the prisoners made him a card at the end. Schoolman and Krige have started a foundation. They want, they say, to continue the work in South Africa's townships and prison.

"If I drop dead tomorrow," says Schoolman, "I'm highly privileged, because if I could make one film that summed up my thoughts about where I was, and what I've been through, this would be it. It is," he says, "a very unusual place to be."

Yes, it is. It's an unusual place, and an unusual film. It's also a powerful reminder that even "a cartload of shit" can sometimes turn to gold.

'StringCaesar' will be screened at Raindance on 3 and 6 October (www.raindance.co.uk)