"I've been in rep all my life", says Derek Jacobi. He means it as a joke but in a sense it's true. For Jacobi, who next week plays Prince Myshkin for the National Theatre in Simon Gray's adaptation of 'The Idiot', has risen to prominence without even once coming into contact with the commercial theatre. Going, as he did, to the National in 1963 after seasons at the Birmingham Rep and the Chichester Festival, he can be seen as part of the first generation of actors to be reared entirely in subsidized surroundings.

After seven years and 25 roles with the National, he gets his biggest chance so far as Dostoevsky's ambiguous innocent. "The basic idea of 'The Idiot'", Dostoevsky wrote in a letter to his niece, "is the representation of a truly perfect and noble man. And this is more difficult than anything else in the world, particularly nowadays." If it's difficult for a writer to make saintly goodness convincing, it's even trickier for an actor to embody it. Where does one start?

"I read the novel", say Jacobi, "about three times before rehearsals. Then when we first started work, people kept saying 'But in the book he does so and so'. After a time one simply had to declare a moratorium on the book and be true to Simon Gray's vision of Myshkin. Dostoevsky's character is a man for all seasons: he changes so often and is so many different things to so many different people--conceivably because the book was originally published in serial form--one can't hope to reveal every facet of the character.

"Simon has had to pick out one through-line and follow that consistently. What he's seized on is the character's goodness and gentleness in relation to his disease. To be an idiot, he suggests, is not automatically to be good; and what we're trying to explore is how far Myshkin's idiocy affects his moral character. Clearly he has the innocence of a child. He's born, as it were, at the stage when most people have had 26 years of their life. It's the old Wordsworthian thing of the innocence that lies about us in our childhood which Myshkin has preserved and other people have lost.

"The problem for an actor, of course, is to be an entirely good man for three hours without being insufferable. One has to live a totally inner life so that the innocence isn't displayed overtly by me but is conveyed by the reactions of the other characters. One has to try and exist on a different plane from everyone else rather than be relentlessly good."

One of the fascinating things about Jacobi's approach to the role is his zealous attempt to get the details of the character's epilepsy right. He's seen films of epileptics supplied by the B.M.A. and the British Epileptics Association, and read papers on the subject. He tells one learnedly that such fits don't necessarily have any visible manifestations, that they can start from a movement or a sound, that they may lead to a state of total immobility.

"Myshkin's fits are of the ecstatic kind. He also knows when they are about to happen. The trouble is Dostoevsky is very specific about their starting with a cry, and in all the films I saw I never came across one of precisely this kind. I asked a doctor about this and he said the cry would be very like the high-pitched wail of a peacock. I'd no idea what this was like so I went off to Holland Park and tried to provoke the peacocks into doing a few cries, but they weren't very obliging."

Jacobi's career to date has been chiefly remarkable for its logical, orderly progression. He was a lauded schoolboy Hamlet. He was a memorable Edward II--quivering, neurotic, and poetically sensitive--in a Marlowe Society production directed by Toby Robertson and with an astonishing cast that included John Barton, Richard Cottrell, Clive Swift, John Bird, Terrence Hardiman, Richard Marquand and John Tydeman. At Birmingham Rep (where his parts included Jimmy Porter, Troilus and Henry VIII), I remember him also as an admirably gauche and nervy Young Marlowe. The bulk of his career, I note, has been devoted to the classics. He agrees; and adds unashamedly that his overriding ambition is to be a great classical actor.

He has stayed with the National while many of his contemporaries have left to explore films and television. Why such constancy? "I like it. I enjoy working with people I know, playing a wide variety of parts and working with a wide variety of directors."

But is there a need for emotional as well as artistic security? Harold Clurman once said of the Group Theatre actors that they were 'yearners seeking a home'. True also of the modern ensemble actor? Jacobi admits that there are emotional ties at the National, to the place and the people. "In a way it might be regarded as cowardice not to move away. But the idea that security makes one complacent is nonsense. People think you need the rigour of being out of work to start questioning, but you don't. If an actor is worth his salt, he is a thinking, reacting mechanism wherever he is."

Jacobi--intelligent, ambitious and privately "untheatrical"--is an interesting example of the new type of actor bred by our big ensembles. Martin Esslin pointed out recently that one of the arguments advanced by opponents of a National Theatre used to be that such institutions inevitably became artistically sterile warrens of 'potbellied Hamlets and post-climacteric-civil-services-status Juliets on pensionable life contracts.' Fortunately it hasn't happened here; and as long as young actors like Jacobi are allowed to rise from the supporting ranks to lead-status, then there's no reason why it should.