In his fascinating acting masterclass on BBC2 last Saturday, Michael Caine told the story of how Jack Lemmon came to work for film director George Cukor after a spell on Broadway. Cukor kept telling Lemmon to do less on camera until the exasperated actor exclaimed: "but I'm doing nothing." "Exactly," the director purred with satisfaction.

There is nothing more excruciating than a theatre luvvie attempting to declaim sonorously to the back of the upper circle when he's five feet away from you in the box across your living room. Theatre and television styles are barely on nodding terms. Sir Richard Eyre, theatre knight par excellence, who just translated his acclaimed stage production of King Lear to the small screen, is the first to admit it.

The walls of Eyre's elegant west London drawing room are bare; he laughs as he explains that he could only afford paintings if he started directing Cameron Mackintosh musicals. Eyre is stimulating company. Manifestly intelligent without being alienatingly academic, he is as likely to throw in a reference to Star Trek as Strindberg. It is this very eclecticism that has made his work so accessible and led Frank Rich, New York's harshest critic, to dub him "the most versatile producer in the English-speaking theatre". How many other directors would be equally at home directing Tumbledown and La Traviata?

Along with his immediate predecessor and successor as director of the National Theatre - Sir Peter Hall and Trevor Nunn - the 54-year-old Eyre is one of very few theatre directors whose name is enough to shift tickets.

But for the moment, his concern is very much with television. "Anything that attempts to put theatre on television is doomed," he says. "You always see it with those clips on televised theatre awards. It's terrible propaganda for the medium of theatre. People at home will think, `that looks such crap, so anaemic'. Saying `just put theatre on television and it'll encourage people to buy tickets' is like saying `give people Vimto to drink and it'll encourage them to like Burgundy'."

"There shouldn't have to be a warning to viewers - `this isn't quite TV drama, this is theatre on TV'. That just exacerbates the notion that theatre is something precious that needs to be protected. By definition, theatre is live communication between audience and performer in the present tense, and it can't be reproduced in any other medium."

So when impressed TV executives saw Eyre's production of King Lear at the National and asked him to transpose it to BBC2, he worked hard to make it a television entity in its own right. "It took a lot more than slipping into the studio and just poking cameras at it," he recalls. "Why patronise both media? TV drama is a singular form. I had to write a screenplay. Cutting it so it doesn't betray its theatrical origins is a challenge. I hope it's a thing that exists as a piece of television."

It does. In the TV version, Eyre has eschewed any sense of proscenium- arch formality and gone for an abstract, even expressionistic style. Dover beach, for instance, is conjured up in a studio billowing with mist. "Once you've made that design decision not to go naturalistic, a lot falls into place," Eyre explains. "It puts all the emphasis on the faces, the light and the way you compose a frame." And it means we can concentrate all the more fully on Ian Holm's deeply moving Lear.

But just why is it that this often difficult and disturbing work continues to have such resonance for us? "It's a play about being human," Eyre says bluntly. "Specifically, it's a play about families, and there's nobody who isn't an expert on the subject. Lear is the patriarch of his family. Fathers of all families are patriarchs and, potentially if not actually, tyrants.

"Then there's the business of expecting love as your due, and what's perpetually fascinating is the child who won't play the game. As Edgar says at the end of the play, `speak what we feel; not what we ought to say'. That's a wonderful exhortation."

When he left the South Bank last October, after a decade in charge at the National Theatre, filming King Lear was one of Eyre's first tasks. (He has since gone on to direct Liam Neeson in David Hare's latest play, The Judas Kiss, which is currently previewing, and in the autumn he has been invited by Meryl Streep and Glenn Close to film them in a version of Schiller's Mary Stuart. He admits he finds it difficult to say no to offers.)

Eyre says now that he gave up the National job "because I foresaw I'd get bored. I knew I wouldn't be able to disguise it, and it would be a poisonous virus that would infect the organisation. I miss the people there, but I can't say I miss that slight tightening of the muscles around the heart every morning as I approached my office. Every day there was a crisis." He once joked that running the National "turned my hair grey".

All the same, Eyre looks back with justifiable pride at his period in charge, during which he personally directed such smashes as Guys and Dolls, The Invention of Love, and the Hare trilogy, Racing Demon, Murmuring Judges and Absence of War. "It doesn't matter how many journalists write that they hate theatre. The National was consistently full for 10 years. That wasn't achieved by focus groups or marketing initiatives. It was achieved by saying that the only thing that justifies our existence is the work on stage. People will only continue to go if the stuff is good."

Eyre jumped straight from the frying-pan of the National into the fire of chairing the government inquiry into the future of the Royal Opera House. He admits the job is a poisoned chalice, "but I don't regret it. I'm very interested and concerned about the world of publicly funded performing arts."

A man who could enthuse Eeyore, Eyre has no time for the critics who accuse him of siding with a government that is doing little to help the arts. "I hope to have a benign influence on arts funding. I've spent the last 18 years saying, `we must have a change of government'. When the government you have prayed for asks, `could you help?', do you say, `Oh no, I don't want anything to do with you, I want to keep my hands clean'? If you constantly remain sitting on the fence, you deserve to get your bum penetrated by it."

`King Lear' is on next Saturday on BBC2 .

`The Judas Kiss' is on at the Playhouse Theatre, WC2 (0171-839 4401) to 18 Apr