Sir Trevor Nunn tells Terry Grimley how pleased he is to be back at the Belgrave Theatre, in Coventry, where his directing career began.

It's more than 40 years since Sir Trevor Nunn began his career as a professional theatre director at the Belgrade Theatre.

Since then he has gone on to emulate his mentor Sir Peter Hall by becoming artistic director of both the Royal Shakespeare Company and the National Theatre, but when he returned recently to direct Scenes From a Marriage, his first production at the Belgrade since the early 1960s, it still seemed like familiar territory.

"I was overwhelmed with memories," he says. "I've been back several times, but going back to start work I remembered so many details. "

The Belgrade was the first of a new wave of postwar regional repertory theatres when it opened in 1958, and its famously adventurous first few years under the directorship of Anthony Richardson has Sir Trevor drawing parallels with the recently extended theatre under its current artistic director, Hamish Glen.

"There's a lot about Hamish that reminds me of Anthony Richardson and vice-versa," he says. "We were doing lots of plays then that hadn't been done before or had only been done at the Royal Court. There was Serjeant Musgrave's Dance, Under Milk Wood, Peer Gynt and Around the World in Eighty Days - that was my first fully-fledged musical.

"It was very innovative and very challenging. Anthony Richardson was setting up educational work, he was really among the first to say there has to be outreach, we have to go into schools. He also introduced after-show discussions. Often we had audiences arranged with unions at the big car factories, and there would be a discussion afterwards.

"That was back in 1962/63. Then you go to the Belgrade at the moment and Hamish has set up a fantastically alternative season with work of a very high order, saying I believe the Coventry audience has the ability to cope with this.

"The outreach stuff and the discussion stuff is already happening, and I'm very enthusiastic about what he's doing."

Mention of once-celebrated 1960s plays like Serjeant Musgrave's Dance raises the question of what determines whether a play which makes an instant impact will eventually prove to be a classic or merely an ephemeral success.

It prompts Sir Trevor to run through some of the long-neglected plays he revived at the RSC, ranging from Dion Boucicault's 19th century London Assurance to an anonymous Jacobean masterpiece, The Revenger's Tragedy. "In every age there are plays which are successful, and then history moves on and the work doesn't survive. But what's also wonderful about what we are able to do in theatre is that we go back and rediscover neglected work."

I wonder whether we might ever see a revival of Birmingham writer David Turner's biggest hit, Semi-Detached, premiered at the Belgrade in the same arts festival to celebrate the consecration of Coventry Cathedral which gave the world Britten's War Requiem and Tippett's opera King Priam.

"I was talking about Semi-Detached only the other day," Sir Trevor says. "It had a wonderful performance at the centre of it in the person of Leonard Rossiter, and when it opened in London it had the greatest actor in the world, Laurence Olivier, in it. But it didn't work because Len had that extra larger-than-life element.

"David Turner wrote the first play I worked on, The Bedmakers, and therefore I found myself in a rehearsal room with him. He was delightful and very funny.

"I remember I went to the first night of Paul Scofield's King Lear with him. He always spoke at the same volume whether he was talking to one person or 12, and I remember him saying [very loudly, in broad Brummie accent] 'I'm not sure, you know, that tragedy on this scale would be possible today, owing to the success of the National Health Service,' much to the amusement of people around us."

Sir Trevor's return to the Belgrade is clearly a coup for the theatre, and Scenes From a Marriage was originally meant to launch the new second auditorium, B2, until events intervened.

With all these neglected plays to revive, I suggest, it seems ironic that he is directing an adaptation of a 1970s TV series.

"Although the writer of that adapted it into a play himself," he reminds me. "But then, when we contacted the Bergman estate we said could we make our own reduction of the TV script rather than use Ingmar's adaptation. That meant a lot of discussion.

"So therefore what we are doing is a new version. In Ingmar Bergman's play he made it just a two-hander. I don't know if that was for budgetary reasons, but it cut out some of the most crucial material, and I'm hoping that people will realise that the new version has restored this."

My admittedly rather hazy recollection of Bergman's TV original is of a somewhat gruelling, forensic examination of a collapsing marriage. Not exactly a frothy evening at the theatre.

However, Sir Trevor points out: "It has got a very bleak and very particular sense of humour. Sometimes that builds up and you have to cut it out and give it its serious level again."

What prompted him to choose it for his Belgrade reunion?

"I remembered it from the TV series and then I remembered it because Alan Howard did the Bergman version of it at Chichester and later in London. Hamish was saying is there any piece you could do something with to open the new theatre, and I agreed the dates but the dates changed. I said to Hamish quite some time ago, what about something that hasn't been done for a very long time and is very, very high quality, and he got excited about it, but very particularly for the B2 space."

Just in case we don't quite appreciate what an exciting venue has been added to the West Midlands theatre scene, Sir Trevor is determined to leave us in no doubt.

"It's just a knock-out! What an incredible privilege Coventry has. It has two really magnificent theatres now. It's always had a very good-sized main house to do main stage performances, but now this other space - if that existed in London there would be a queue of companies trying to use it absolutely 52 weeks a year, 100 years a century. It would be constantly in demand.

"It's like the Cottesloe at the National. We have to be using it in its end-on format because that's how the play is written. It would subvert the play if we did it in traverse or in the round."

Just as Ingmar Bergman directed his then wife Liv Ullman in the original TV version, Sir Trevor has cast his own wife, Imogen Stubbs, as one of the marriage partners. Completing a tight family unit, the other one is played by Iain Glen, brother of Hamish.

"Iain and Imogen have actually known each other for the greater part of their lives," Sir Trevor points out. "They knew each other as children and went to RADA together. Iain got the gold medal in the year that Imogen got the silver.

"I've worked with Iain before and I've worked with Imogen before, and it's just a very good opportunity for people who know each other to work together."