Although he abandoned his brief acting career years ago - "I didn't travel far, but at least I learnt from the journey" - last year Richard Eyre had an uncredited cameo role in a National Theatre production. Nobody saw him. In fact, his appearance was entirely fictional. He was conjured up as a character in Violin Time, a deliriously ludicrous comic monologue by Ken Campbell, Britain's comic maestro of theatrical anarchy. Campbell spun a yarn in which Eyre warned him of a dire predicament: "You are the victim of Famation of Character... They've got you on the hamster/ gerbil treadwheel now. It means your next show has got to be better than they said your last one was." Campbell is still spinning, but two weeks from today Eyre will open his production of Tom Stoppard's The Invention of Love, his National Theatre swansong, before stepping quietly off the wheel of success.

Since succeeding Sir Peter Hall as artistic director of Denys Lasdun's South Bank landmark, he has run a building with a staff of 800, and overseen 189 productions, including 65 new plays and seven West End transfers, 22 of which he directed himself. Several of these have crossed over to Broadway and beyond, prompting Frank Rich, New York's most famous theatre critic, to describe him as "the most versatile producer in the English- speaking theatre". He has also found time to give 32 Platform performances, write Utopia and Other Places - an eloquent account of his childhood combined with astute essays on theatre - nip across to Covent Garden to make his opera debut directing Verdi's La Traviata (at the behest of the late Georg Solti), pick up a knighthood and listen to The Archers. Not bad for 10 years' work.

Playing a character, as underlined by his (non) appearance in Violin Time, chimes eerily with his feelings when he began the job. In a rare, published excerpt from his diaries he described himself as "performing an act called Richard Eyre". In the midst of rehearsals for the Stoppard play, he evokes his first year with a suitably theatrical analogy. "What you say about an actor who is inadequate to a role is that they're over- parted. I felt over-parted at the beginning. I found it very, very tough, partly because I thought I could do the job before I started it but then, when I did start, it seemed so much larger, more lonely than I'd imagined." A smile creeps across his lean, serious face. "I was well-inclined to back out. It was a toss-up as to whether to continue with the danger of ignominious failure or to resign in deep shame. Cowardice won out," he laughs, sheepishly.

In a recent, rather bitter tirade, playwright and director Peter Gill attacked Eyre's programming, suggesting that he'll be remembered solely for Guys and Dolls. It wasn't meant as a compliment but thousands of delighted theatregoers would see that as a more than fitting monument. In the face of successive standstill Arts Council grants, Eyre remounted this money-spinner to a whirlwind of praise. Gill's protestations mark him out as one of a startlingly small number of dissenters. The first year was tough, as Eyre found himself having to carry out inherited job losses following a pounds 3.5m cut in subsidy, but by the end of the Eighties he'd relaxed and his tenure is now routinely described as "a golden era".

That's no mean feat. He has had to fill three auditoria, each housing three plays at any given moment. That means nine productions in the repertoire plus three in rehearsal, another 12 in the planning stage, plus touring, developing new writing in the National Studio, the burgeoning education department, not to mention the West End and Broadway. Most of his critics resemble reviewers of anthologies who complain about the exclusion of their personal favourites. A lack of classic European work has been noted, while others have frowned at the accent on American writing. Notoriously sensitive to criticism though he is - "journalists are not slow to point out a run of failure, by which they mean a couple of shows which aren't up to scratch" - Eyre is sanguine about it. "By definition, the National Theatre has a sort of exemplary role. It's a loaded definition I know, but there's an expectation that it offers the best of theatre and satisfies all constituencies. It's castigated for not being in the vanguard of new writing, or for not upholding the classical tradition. It's expected to tour, or to present the very best in scenic design yet on the other hand not to be lavish in expenditure on design, and so on and so on."

As his legion of supporters will tell you, Eyre, an almost diffident character, is the last man to sing his own praises. Much of the credit for the National's success, however, is down to his taste. Genista McIntosh, until recently his executive director, describes this urbane man who is notably short of small talk in surprising terms. "A real showman, although Guys and Dolls is an unusual example of his work, which is careful and austere in the best sense. He's a very entertaining and funny man who enjoys being entertained, which is not in any way to make him sound superficial. His great skill as a producer is that his rigorous and tasteful approach is always on the edge of being subverted by a sense of razzmatazz."

Former associate director David Hare highlights Eyre's theatrical archeology, the policy of resurrecting neglected classics which resulted in great evenings like Rutherford and Son and Absolute Hell. He also pinpoints a historical change of direction. "By the time he arrived it was clear that the staple post-war repertory wasn't working with the public. Richard is responsible for the invention of a new repertory, a multiplicity of different kinds of theatre." He's referring to the highly successful gambit of drawing in smaller companies with co-productions, companies like Jatinder Verma's Tara Arts, Theatre de Complicite, Cambridge Theatre Company and Gloria. Eyre sees this more simply. "I always quote George Devine: `Policy is who you work with.' What plays you do, who's going to direct them, who's going to act them, design them and so on. Those decisions are my job." One of the achievements of which he is most proud was the mounting of Hare's 1990-93 state-of-the-nation trilogy (Racing Demon, Murmuring Judges and The Absence of War). "His work as a director has been overlooked," says Hare. "When the trilogy was performed over a single day, the standard of acting at the end of the evening was as high as it had been in the morning." Patrick Marber, Steve Coogan's long-term comedy collaborator turned author of the hit National Theatre productions Dealer's Choice and Closer, feels he owes his playwriting career to Eyre. He also admires Eyre's directing skilll. "I've never seen a badly acted Eyre production." This probably stems from the relaxed rehearsal atmosphere observed by his regular production photographer John Haynes, who earmarks Eyre's loyalty and intuition. It is those qualities which led him to create a family of directors like Declan Donnellan (Fuente Ovejuna, Angels in America) and Nicholas Hytner (Ghetto, The Madness of George III) and to exclude certain directors who have gained popularity elsewhere but who doesn't meet his exacting standards. "It's only in the interest of self-preservation," he says, self-deprecatingly. "You can only work in good faith. Trying to second-guess the audience is inevitably corrupting. If you say, `I know it's not very good but audiences will come', then it corrodes you." This article of faith has been force to battle with the need to balance the books. "Maybe I should have been more reckless," he muses, "but one of the reasons the National has survived is that I've recognised that I do something well, but other people do things better."

His blend of confidence and caution creates the contradictory image of a pragmatic idealist, borne out by his articulate public voice. "Twenty years ago people were not saying, `Oh, isn't theatre a fucking bore'. People weren't being paid by the Sunday Times to write long pieces about Why I Never Go To The Theatre. So I felt I had to act as an advocate for the medium and the only way you can do that is actually on the stage. You've got to find the work that says, `Look, you don't have to go to theatre, no one's obliging you to, but if you do go, there's an experience you can't have anywhere else'. Yes, bad theatre is what you say it is but look at how many bad movies or bad bands there are. So I got very interested in Robert Lepage's work, in Theatre de Complicite, in encouraging David Hare to play to his strengths, or Stephen Daldry doing An Inspector Calls. They're all about people making the medium as expressive as possible and making an argument for theatre as a medium that can stand alongside Damien Hirst or Trainspotting and you could just hold up your head and say, `It's not what you say'."

Nevertheless, he concedes that bad theatre is peculiarly difficult to watch. "I know people who absolutely hate even being invited to join a community. The point about theatre is that you go in as individuals and end up as a community. If you want to dissent from that you can't do it invisibly. In a cinema, unless you go and assault the projectionist, whatever you do is not going to affect the performance. It's a delicate chemistry but, when you get real theatre, the very air becomes charged up. You can go into Lear and take the temperature before it starts and do so again at the end and it's as if every molecule has been electrically and emotionally charged. Now, I like that. Some people don't."

That kind of non-defensive confidence scotches any notion that he might go gently into that good night. Solti's death has robbed us of their projected Pelleas and Melisande but he will film his powerful King Lear with Ian Holm for the BBC. He is also discussing a series on theatre in the 20th century, not to mention directing Liam Neeson as Oscar Wilde in The Judas Tree, Hare's new play at the Almeida next year. He is also considering writing fiction, "to the horror of friends of mine who are novelists". There certainly won't be much time for regrets as he watches Trevor Nunn take up the reins. "What I won't miss is what I call `the blocked loo syndrome'," he confides, chirpily, "where everything ends up on your desk up to and including someone coming round and saying, `There's a loo blocked in front-of-house'. To which you have to say, `Well, get out the plunger'."

`The Invention of Love' previews from 25 Sept at the National Theatre, London SE1 (0171-928 2252). Richard Eyre will be in conversation at the National on 29 Sept