When Milo Rau first saw the Ghent Altarpiece, one detail leapt out at him. While Hubert and Jan van Eyck’s 15th-century masterpiece is often admired for the ambitious scale of its Lamb of God scene, the Swiss theatre director zoomed in on Adam’s sunburnt hands. “They are red because the model came directly from the fields to be painted,” he says the morning after the premiere of his new Altarpiece-inspired production, Lam Gods. “It’s art as a collective process, made with common people.”

It is a philosophy Rau is attempting to apply to an institution he has long decried as elitist and preoccupied with the classics: the city theatre. Starting this season, the 41-year-old is now the director of one of Belgium’s foremost venues, NTGent, and has kicked things off with a community-oriented production. Lam Gods is the Dutch name of the Altarpiece, on display at St Bavo’s Cathedral near the theatre, and Rau brought 21st-century models to the stage to reinvent, through a video installation, the biblical figures and characters painted by the Van Eycks – Ghentians from all walks of life.

“I wanted to have a song for the common man, so everybody understands: this is my theatre, my city,” Rau says. While he made a name for himself with provocative independent work that questioned reality, from re-enactments of international trials (The Congo Tribunal, The Last Days of the Ceauseșcus) to explorations of paedophilia (Five Easy Pieces) or the Rwandan genocide (Hate Radio), he felt as if he’d “come to a limit” with his production company, the International Institute of Political Murder. “And I thought: like Godard says, you can only criticise bad films by doing a better film.”

‘You have to be international and local at the same time’ … theatre director Milo Rau before a reproduction of the Ghent Altarpiece. Photograph: Michiel Devijver

To this end, in May he published the Ghent Manifesto: a list of 10 rules that all NTGent productions are required to follow. “It’s not just about portraying the world any more. It’s about changing it,” the first reads. It dictates that classic plays are forbidden, unless the source text represents no more than 20% of the final product; at least two actors on stage must be amateurs; each production must tour internationally, among other requirements.

Rau acknowledges, with a laugh, that an artistic manifesto sounds slightly old-fashioned. “But technically, when you follow the manifesto, you will have the city theatre you want. The structure will change.” La Reprise. Histoire(s) du Théâtre (I), Rau’s recent, stunning production inspired by the murder in Belgium of a young gay man, Ihsane Jarfi – which combined a realistic re-enactment of the crime with illuminating commentary on the process of adapting it for the stage – was a “totally pedantic” demonstration that the manifesto can work, the director adds wryly.

Rau’s Five Easy Pieces explored paedophilia. Photograph: Phile Deprez

The rules are a perfect example of the mix of artistic bravado and marketing savvy that has kept Rau firmly in the European spotlight. He doesn’t mind controversy: spontaneously, he lists the handful of mini-scandals that have broken out since his arrival in Ghent. First, there was the dissolution of NTGent’s permanent ensemble, in order to ensure more diversity on stage (“If you want to have a more diverse ensemble, you have to say bye-bye to the old men”). Then Rau ran an ad calling for former Islamic State fighters to appear on stage in Lam Gods, which prompted a debate all the way to the Belgian parliament.

In the end, NTGent apologised for the ad and Rau opted not to cast former jihadists, although one appears in video. Instead, Lam Gods features Fatima Ezzarhouni, the Muslim mother of a Belgian Isis fighter who was killed last month. “What’s important is the milieu they come from. Suddenly you understand: these are the sons of our country. It’s a Belgian family story.” At the end of her powerful, dignified testimony, Ezzarhouni donned a veil to represent the Altarpiece’s Virgin Mary – before pulling out of further performances.

Regardless, Rau praises Belgium’s open-mindedness, and says one controversial scene in Lam Gods – in which one of two couples appearing as Adam and Eve simulate sex in front of a semi-circle of choir children, who double as the Altarpiece’s angels – wouldn’t have been possible in many countries. (In truth, the youngsters mostly looked bored.) He believes the dysfunctional Belgian state, regularly paralysed by tensions between the Flemish and Walloon communities, has made “civic society extremely strong. For generations, they’ve learned to do it themselves, somehow.”

Artistically powerful, politically shrewd … Lam Gods directed by Milo Rau. Photograph: Michiel Devijver

And Lam Gods, which involved meeting hundreds of amateurs and was informed by Rau’s background in sociology, is a tribute to a diverse Ghent. It brings together lifelong inhabitants, immigrants, an NTGent cleaning lady and even a shepherd who shears a sheep on stage – a gesture that is, again, artistically powerful and politically shrewd. Ghent’s mayor, the socialist Daniël Termont, gave a speech to open the season; Rau was keen to avoid the mistakes he saw his friends and peers Chris Dercon and Matthias Lilienthal, whom he calls “internationalists”, make in Germany. Dercon, a former Tate Modern director, was forced out of Berlin’s Volksbühne after protests against attempts to reinvent its identity with a global outlook, while Lilienthal will leave Munich’s Kammerspiele in 2020. “My humble interpretation is that they didn’t embrace the cities. You have to be international and local at the same time.”

Rau is doing just that: rule nine says that at least one production per season “must be rehearsed or performed in a conflict or war zone”, and for Oresteia, a production inspired by Aeschylus that is scheduled for next April, he will travel with his team to Mosul, in northern Iraq, a former Isis stronghold. He acknowledges the difficulty of the project, but is unfazed. “I prefer to invest €20,000 in Mosul or in Bukavu than in Zurich or Berlin. When we did The Congo Tribunals, there was no cameraman, no production company there … You train people. Some say it’s war tourism, you just want to have your fantasy of being in danger. After doing it for 15 years now, I can say: it makes a change. A small change.” Or a giant leap, if Rau’s bold vision for theatre comes to fruition.