Click here for an interview with Simon Callow's co-star Alfie Allen

A half hour's chat with the ebullient Simon Callow is like circuit training for the brain. Such is the seamless flow of perfectly expressed ideas, opinions and insights from the silver-tongued Mr C that you're left red-faced and panting in his wake, as you struggle to keep up with the intellectual pace he sets.

Callow is one of these hyperactive, high achieving multi-taskers who protest that their incessant activity masks an inherent laziness.

"I have to whip myself into action" he announces.

Such claims are to be met with some scepticism. When asked to calculate the number of projects with which he is currently juggling Callow replies, deadpan: "Twelve serious ones and twenty-five not so serious."

Playing the role of psychiatrist Martin Dysart in this production of Equus means that Callow is permanently on his feet. Yet after a hard day's rehearsal, he thinks nothing of spending another three hours working with the writer of a screenplay he hopes to direct.

The breadth of Callow's achievements and accomplishments is astonishing. A successful stage and screen actor for thirty years - who can forget his larger-than-life Gareth whose untimely demise provides the funeral in Four Weddings and a Funeral ? - Callow has also directed opera, films and plays.

He recently steered Jerome Flynn towards an amazing performance as Tommy Cooper in Jus' Like That and his staging of Willy Russell's Shirley Valentine was a major factor in the success of the show.

He's also an acclaimed biographer, notably of such similarly outsize personalities as Orson Welles and Charles Laughton, the subject of a play he's written with Timothy Spall envisaged for the leading role.

And whenever he has an idle moment, Callow is likely to be reading - either for pure pleasure or for the literary pages of The Guardian where he is a much respected reviewer.

"It's a standing joke on the paper that I never get sent books of fewer than seven hundred and fifty pages." laughs Callow.

Callow is associated with Equus in a number of ways. Not only was he a friend and colleague of John Dexter who directed the original 1973 production but he was also the first actor to perform as Mozart in Amadeus, the worldwide hit play written by Peter Shaffer, the author of Equus. And there is another connection.

"I happened to be in the audience for the very first preview of Equus" reveals Callow. "I vividly remember Peter Firth in all his golden radiance as Strang and Alec McCowen as Dysart emerging out of the darkness.

"It was only six years later that I came to play Mozart and I realised that Peter (Shaffer) writes plays that are kind of fail-safe. I'm interested in how we communicate with an audience and when you go on stage in a Peter Shaffer play, there's always the possibility of something unique happening. In the two years I played Mozart, there was only one performance when it didn't ensnare the audience. It was when Margaret Thatcher, then the Prime Minister, was in the theatre and people were too busy watching her response to concentrate on what we were doing on the stage."

When an actor has such a strong recollection of another performer's interpretation of a role he is now playing, it can't be easy shaking off that memory.

"Alec and I are two very different types of actors" Callow points out. "He was like a greyhound bursting out of the traps, relentlessly pursuing these ideas in a way that gripped you by the throat. And Dysart is not a 'character' in the sense that Falstaff is - you wouldn't give him a limp and a Northern Irish accent, for example. He's like a filter, a conduit for thought and emotion. I've been thinking about how to play him by referring to Paul Scofield, who was Salieri in that first production of Amadeus. Paul has a far-flung empire of expressions which he carries within him and he inhabits them all."

At the same time, Dysart is confronted in the case of Alan Strang by a very human, recognisable dilemma.

"Dysart is a reflective man, a wonderful healer. He has certain gifts but it is these very gifts which he has come to doubt and discard. I think that there's a large vein of melancholy within him and this melancholy is connected with his longing for what he sees as the intuitive values of the classical Greek civilisation."

On the face of it, Dysart is a good man, doing good work as he sets about mending broken and wounded minds. Yet, during the course of the play, he comes to doubt his own mission.

"Dysart has had to deal with some of the most terrible casualties of modern life" says Callow. "He has removed their pain, he has talked away their terrors but he now wonders if he has not excised something that is central to who they are. He feels that he is returning people to normality, solely that they can become model citizens of the industrial society once again."

By a quirk of fate Callow was later to work closely with John Dexter, whose staging of Equus he had so muich admired. In the late 1970s, Callow joined the National Theatre to play Orlando in Dexter's production of Shakespeare's As You Like it and was reunited with him on Brecht's The Life of Galileo with Michael Gambon in the title role. And there could have been a third collaboration.

"It was actually John (Dexter) who originally cast me as Mozart and Peter Hall was kind enough to honour that arrangement when he took over the production. I was fond of John, although he could be a brute in many ways. He never did me any harm and he believed very strongly in my talent. We were always going to work together; there were plans for me to play Richard III and Hotspur for him and I was going to play Edgar
in Gielgud's Lear at the National. But John talked to the press about it, Gielgud took fright and the production never happened."

Even in those days Callow was pioneering the art of the multi-tasker. Not content with playing Mozart and Orlando, among other commitments at the National, he undertook the daunting challenge of learning and performing Shakespeare's Sonnets, all one hundred and fifty-four of them, in a programme which he'll revive in a shortened form for the Shakespeare Festival at Stratford, Ontario later this year. Also on the Callow horizon at the moment is a production of Mozart's The Magic Flute for the Holland Park Festival in West London and the third volume of his much praised biography of Orson Welles.

"I'm just a boy who can't say no" he confesses, slightly paraphrasing Rodgers and Hammerstein. "People ask me to do things and I find it hard to turn them down. I love acting deeply but it's agony for me to have to wait for anything and so I can't bear that side of the job that entails you waiting for somebody to offer you work. I feel obliged to achieve things all the time."

Callow observes with more of a kind of puzzled resignation than regret that "I'll know that I'll barely merit a footnote in the history of the English Theatre." He continues "I've done no Ibsen, Strindberg and Chekhov, only three Shakespeare plays and very little Restoration comedy. I envy people such as Simon Russell Beale, Alex Jennings and Tony Sher their Classical careers. I want to play Shylock and Lear and tackle Titus Andronicus again, a part I first played when I was about twelve."

Nor is there much consolation from television. Despite Callow's tour de force on stage as the great writer in The Mystery of Charles Dickens, a role he repeated in an episode of Dr. Who, he is strangely overlooked by casting directors looking to fill the ranks as the next adaptation of a nineteenth century adaptation arrives before the cameras.

"I think that I'm perceived as a bit of a nineteenth century relic and since they're doing all they can to modernise Dickens, I'm not on their list."

Not that Callow is remotely given to self-pity at this inexplicable neglect by some sections of the industry. He is having too busy and too pleasurable a time. Before Equus rehearsals began in January, he'd done a quick tour of the Baltic, attending the Stockholm premiere of Arn The Knight Templar, a mediaeval epic in which he plays "an old holy man" whose scenes were filmed "in a castle outside Edinburgh".

From Sweden it was on to Moscow and St. Petersburg for the Russian section of a programme on composers which Callow presents for Australian television. Other red letter events in Callow's 2007 diary were a pilot for American television which he made in Los Angeles and The Chemical Wedding, an intriguing project about the occultist Aleister Crowley, co-scripted by Bruce Dickinson, once of Iron Maiden. And Hollywood keeps calling. Callow enormously enjoyed his stint on the cinema version of The Phantom of the Opera which led to his succeeding Michael Ball as the scene-stealing Count Fosco in Lloyd Webber's West End musical The Woman In White.

Perhaps Callow suffers - if suffer is the right word - from the traditional British suspicion of versatility and from the unease felt by some directors at having in their acting company, a colleague of equal if not superior talent.

Whatever the reason, at least it ensures that Callow will always keep us guessing. One thing you can predict with certainty - the unpredictable Mr C will be full of surprises. You never know where he'll strike next and long may he continue to do so.

Equus is at Milton Keynes Theatre from Monday, March 17 to Saturday March 22.
Tickets are subject to availability. Proof of age will be required when booking if you wish to take advantage of an Arts Council England (ACE) special offer, which offers tickets to those aged 25 and under for just £10.
Box Office: 0870 060 6652 (booking fee)
Click here to go to Milton Keynes Theatre's website.