In a little room underneath Guildhall in London, two of Britain's greatest Shakespearean actors are in a state of elongated thespian ecstasy. “First time I've ever touched one of these,” says Simon Russell Beale, his whisper tremulous with emotion.

“Me too,” adds Emma Fielding, who played Lady M to Beale's Macbeth in a riveting production of the Scottish play three years ago.

“It's the most influential book ever!” Russell Beale declares. “And yet the irony is that you can't trust it. Not 100 per cent.”

The book they hold with such awe is what's known as the First Folio. Or, to give it the title that its compilers, John Hemminge and Henry Condell, devised in 1623, Mr. William Shakespeares Comedies, Histories, & Tragedies. Running to 900 pages and including 36 plays, it is the most authoritative Shakespeare source that has come down to us, and - in the lamentable absence of a single manuscript in Shakespeare's hand - the only reliable source for half the plays. “Look!” Russell Beale cries. “Here's the original cast list for Romeo and Juliet. They're all here: Richard Burbage, Nathan Field, Gwyneth Paltrow ...”

Only 1,000 copies of the First Folio were printed, and only 228 are known to exist today. If you find another in your attic, you've struck gold: the last time a First Folio came up at auction, it went for £2.5 million. And of those 228, more than a third are hoarded in one place: the Folger Shakespeare Library in Washington DC. Why one institution should need 79 copies of one of the world's most precious literary treasures is a matter of some debate.

But Russell Beale and Fielding are not poring over one of the Folger's First Folios. No, this mint-condition volume belongs to the city where Shakespeare plied his trade. And, fittingly, it's in a library just yards from where he lived at the height of his fame - in Silver Street, near Cripplegate. The City of London Corporation acquired it early last century, and it is kept in the Guildhall Library, permanently available for perusal by serious Shakespeare scholars. But in honour of Shakespeare's 444th birthday (tradition holds that he was born on April 23, 1564), the Corporation has allowed two of his finest modern interpreters to get their hands on the volume and explain its importance to The Times.

So, Mr Russell Beale, justify yourself. Why can't we trust the First Folio? True, it was compiled more than six years after Shakespeare's death. But the task of sifting through the hundreds of authorised and unauthorised copies of his plays circulating throughout theatreland in the 1620s was undertaken by two actors who were so close to the Bard that they were mentioned in his will.

“Well, let's take Hamlet,” Russell Beale explains. “The First Folio was so obviously an attempt to make a canonical record of Shakespeare's work that you instinctively feel that a great deal of thought must have gone into the problem of what to include. Yet important passages in Hamlet are missing from the Folio but found in the Quartos [earlier texts of the plays]. You don't know whether this is Condell and Hemminge saying: ‘We are going to print all we've got' - which isn't true, because one Quarto has a more complete text. Or whether they are saying: ‘We're printing it as it's performed now, in 1623.' Or whether they are saying: ‘This is the way it was performed when Shakespeare had finished tinkering with it'.”

So for today's actors, the First Folio is as tantalising as it is inspirational. But does Russell Beale really think that the greatest of all dramatists was content to have his plays rewritten in rehearsals? “Absolutely,” he says. “The process of changing an author's text into a performable entity is something that has always gone on. I remember rehearsing Charlotte Jones's Humble Boy; Diana Rigg had to say a line about water spaniels. In rehearsal, she suddenly said to Charlotte: ‘Water spaniels do not do this.' So the line was rewritten.

“I'm sure that sort of negotiation went on with Shakespeare, too. I can't believe that the first Macbeth and Lady Macbeth didn't turn to Shakespeare and say: ‘Look, Will, did she have a baby or not?'”

“It's the obvious question,” Fielding adds. “And if she did, where is it?” So why didn't Shakespeare clarify that in the dialogue? “Who knows?” Russell Beale says. “He probably replied: ‘Look, the baby died, OK? Just say the lines.'”

Russell Beale and Fielding maintain that the First Folio is the book that “gave us our language”. “We still speak like this,” Russell Beale says. “The proportion of words that Shakespeare uses that have the same meaning today is very high.”

“Especially compared with contemporaries such as Ben Jonson or John Ford,” Fielding adds. “Set beside them, Shakespeare's language sounds remarkably modern to us.”

“Mind you, individual words still cause us trouble,” Russell Beale says. “When I played Hamlet there was debate in the press about my weight. Because, of course, no Hamlet should be plump! Yet in the play there's that description of Hamlet as ‘fat and scant of breath'. When I point that out, critics say: ‘Yes, but in this context fat means sweaty.'

“So I say: ‘OK, how about the line in Julius Caesar: ‘Let me have men about me that are fat'? To which they respond: ‘Oh no, fat definitely means fat there!' I've got this theory that Shakespeare looked at Burbage, who by then had been knocking around for decades, and thought: ‘Ooh, he's put on weight!' So he added the fat line into Hamlet.”

Which raises the question of how much Shakespeare tailored his writing to the talents of the actors in his company, the King's Men. Burbage's emotional range and power obviously inspired the creation of great tragic figures such as Hamlet, Lear and Othello. But what of smaller roles?

“Well, the coming and going of clowns within the company, and the effect on Shakespeare's writing, is fascinating,” Russell Beale says. “When Shakespeare first came to town in the 1580s the great clown was Tarlton. It's said that the very sight of his face set people laughing. Just like Frankie Howerd.”

“He could do ‘the nothing' thing, you mean,” Fielding adds.

“Then Will Kempe came along,” Russell Beale continues. “But he left the company in 1599 to do his famous morris dance from London to Norwich. Possibly the worst career move in history, just before Shakespeare wrote all his biggest hits. You can imagine him a few years later, thinking: ‘Er, whoops'.”

“It was a kind of Pete Best moment,” Fielding quips.

“But when Kemp was replaced by Robert Armin,” Russell Beale continues, “Shakespeare's clowns became very different. Feste in Twelfth Night and the Fool in Lear are much more elegiac, almost tragic, figures.

“What we don't know, however, is for whom Shakespeare wrote parts such as Cleopatra and Lady Macbeth - those hugely sexy women who, in the Elizabethan theatre, would have been played by boys.”

They must have been very worldlywise boys, to rise to those transvestite theatrical challenges so convincingly. “Yes,” Russell Beale agrees. “You have to wonder how they played the parts - even if they were as old as 18, because boys' voices broke later in those days. In As You Like It, for example, it's the girls who hold the play together.”

What of the stage directions in the First Folio? Are they Shakespeare's? Do they represent what he wanted in terms of effects and scenery? If so, should today's directors be paying them more attention than they do?

“Well, people think that the most elaborate stage directions must have been Ralph Crane [the scribe who prepared fair copies of the plays for the printer] describing what had happened in Globe performances, rather than Shakespeare's own instructions,” Russell Beale points out. “Look at this from The Tempest: ‘Thunder and Lightning. Enter Ariel, like a harpy; claps his wings upon the table; and, with a quaint device, the banquet vanishes.' Many scholars argue that no dramatist would have been so irresponsible as to write something as vague as ‘with a quaint device'. They think Shakespeare would have been much more specific. I'm not so sure. I think of Tom Stoppard in Jumpers, where he has a woman swinging on a trapeze, acrobats all over the stage, and then he simply writes the stage direction: ‘George's study appears.' As if to say: ‘Over to you, Mr Director!'”

Holding the First Folio, reading from the very type that Shakespeare's own actors would have perused, Russell Beale and Fielding clearly feel what Tony Blair would undoubtedly describe as “the hand of history” on their shoulders. But how much do they feel it anyway? Do Shakespearean actors at the top of their trade sense a direct lineage back to those privileged thespians treading the boards with the Bard in the early 1600s? And if so, isn't it hugely intimidating?

“Well, perhaps Olivier, in his private moments in bed, thought that he was in a direct line from Burbage,” Russell Beale says. “But I think young actors are already too scared of Shakespeare. Of course these plays are difficult to do. But so is Stoppard. So are TV sitcoms. They all have their disciplines and challenges. When I talk to students about doing Shakespeare, I say: ‘Don't worry, the rules are actually very simple - and half the time we break them anyway.'”

“When you perform Shakespeare, you do feel as if you are picking up the baton in a very long relay race,” Fielding adds. “Especially if you do a big role such as Viola in a place such as the Royal Shakespeare Theatre, which has so many ghosts. But I do feel that the ghosts are benign. They don't say: ‘Play that part if you dare!' It's more: ‘Now it's your turn - good luck.'”

Reluctantly, Russell Beale hands back the First Folio to its keeper at Guildhall. You feel that he would like to stuff it under his coat and smuggle it over the river to the National, where he could ceremonially flourish it to settle contentious issues in rehearsal. “To my mind it has settled one point already,” he says. “It proves that Shakespeare's plays were written by Shakespeare! Otherwise Condell and Hemminge - producing the first edition - would surely have taken the opportunity to say: ‘Actually we were kidding you for 30 years. Shakespeare wasn't Shakespeare after all - he was the Earl of Essex!'”

Times recommends

• Babyboomers still partying on in their sixties
• Do I have a chance of finding love again?
• Video: How to do party make-up