Call it coincidence or maybe even fate. Last year Trevor Nunn was on holiday in Tuscany, ploughing through On the Origin of Species after he’d been asked by the Darwin Society to stage something to mark the 200th anniversary of the naturalist’s birth. Suddenly memories of an American play called Inherit the Wind hit him. That involved the “monkey trial” of 1925, when the state of Tennessee prosecuted a young teacher for introducing schoolchildren to the heretical Darwin. The play had special significance for Nunn, as he’d chosen part of it as his audition piece when he arrived at Cambridge 50 years before. Thanks also to his lead actor, “a dark, curly-haired, precocious boy called Stephen Frears”, he convinced senior undergraduates that, yes, he’d make a good student director — and so began a career that would take him to the helms of both the RSC and the National.

But where should he stage Jerome Lawrence and Robert E. Lee’s play? Who should succeed Frears as the defence counsel, a thinly disguised Clarence Darrow? Why, the Old Vic and its artistic director, Kevin Spacey, who had played Richard II there for Nunn. So Nunn e-mailed the actor, to find that he was at a conference in Russia and, asked by his hosts for a monologue, had just delivered a Darrow speech. “Let’s talk,” replied an astonished Spacey. And so they did, with the result that a play whose subjects include the still-topical battle between evolutionary ideas and fundamentalism is about to open at the Vic.

When we meet between rehearsals Nunn, 69, looks satisfied not just with that day’s work but with life in general. “I feel great,” he says. “I run up escalators and don’t stop or gasp at the top. I walk, I swim, I’ve lost three stone.”

So has giving up his post as National supremo, which he did in 2003, proved a bane or a boon? “Well, you make these deep friendships, you develop a shorthand communicating with people, so when you lose that contact it’s a big shock to the system. But, oh, I can actually have a lunch break, I don’t have to do papers and phone calls the entire weekend. So, yes, the freedom is absolutely exhilarating.”

That freedom has recently brought us work varying from a majestic King Lear with Ian McKellen to Cyrano with Joseph Fiennes to, oh dear, Gone With the Wind. That musical’s failure still dismays him, though he finds solace in the fact that it has excited the Koreans enough for it to be opening over there. But what Nunn’s CV continues to demonstrate is his love of variety, his belief in diversity.

That meant he was accused of populism when he was running the National. Why was he reviving musicals such as Anything Goes and South Pacific? Well, as he points out, the first was staged in tandem with a Love’s Labour’s Lost full of intimations of the First World War and the second wasn’t just criticised by some reviewers for being too serious but was followed by Tom Stoppard’s Coast of Utopia trilogy. What was essential, he felt, was to widen the National’s appeal and audiences: “I thought we can’t only do what highly educated readers of the arts pages want to see. We’re subsidised and everybody in the country pays taxes, and therefore a national theatre has to be available to everyone.”

Inherit the Wind, which boasts 23 actors plus six children plus 34 “supernumaries” for crowd scenes, plays to Nunn’s strengths at mounting big shows in big theatres. It is a play which, he thinks, isn’t topical just because so many Americans believe in Creationism and want it to be taught alongside or instead of Darwin in science classes. It’s timely because here two libertarian ideas are in conflict with one another. “One side is saying you can’t teach a generation of students and censor one vital, fundamental sense of who we are and where we come from. The other is saying that there are people from other traditions for whom Darwin is a sensitive subject, and it’s more important to keep a community happy than allow teaching that might cause distress.”

What next for the man who keeps his watch ten minutes ahead of time that to ensure he’s never late for appointments, yet looks a bit hurt when I call him a workaholic? Next month Nunn is off to Broadway to restage the production of Sondheim’s A Little Night Music that opened in the tiny Menier and moved into the West End. He’s writing his memoirs and about to prepare the lectures he’ll soon give as the new Cameron Mackintosh visiting professor of theatre at Oxford.

Film is the medium in which he has had no great success. He had set up a small company to develop screen projects when the National asked him to take its top job in 1997. The theatre will, it’s clear, remain his focus, “but I’d love to have another go at a film". And, given the energy of the 69-year-old who hares up escalators en route to orchestrating the massed bigots of 1925 Tennessee, that’s perfectly plausible.

Inherit the Wind is previewing at the Old Vic, London SE1 (0844 8717628) and opens on Oct 1

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