In 1937, Orson Welles presented Shakespeare's Julius Caesar as a terrifying vision of the consequences of fascist rule, in a production menacingly subtitled Death of a Dictator. It was a breathtaking update of a play that for centuries has lent itself to both political and personal manipulation. In the 1770s, Americans forgave Shakespeare for being a Brit and used it as an inspirational text for US independence. An Italian production in the 1990s featured endoscopes, mechanical animals and two anorexic women playing Brutus and Cassius. This year, a political agenda predominates once more. With the consequences of the Iraq invasion hitting the news daily, it is - according to the director Deborah Warner - "a screamingly good moment to look at the play".
As Warner prepares to reveal her own interpretation at the Barbican, she is not surprised that another production - featuring Denzel Washington as Brutus - is opening in New York. "In times of crisis, we go back to the strong texts," she says. Yet, despite being struck by Julius Caesar's renewed relevance, she is refusing to yield to the temptation of creating overt political parallels. "The play has suffered in the past because people have tried to swing it in one direction or another," she protests. "So I'm not, for instance, trying to compare Caesar with Bush - it just wouldn't work. Caesar was a brilliant strategist. Bush rigged a couple of elections."
On the other hand, for her modern-dress production, Warner believes that "the Blair/Brutus comparison is very interesting. What this play deals with is people making difficult decisions, and being forced to go through with the consequences once their initial conviction has gone. It is left open whether Rome has been served positively or negatively by Caesar's death. But what is clear is that when Shakespeare looks at the consequences of violent conflict, he reveals that the act of murder is always catastrophic."
Theatre companies are now so desperate to be politically relevant that it can only be a matter of time before A Midsummer Night's Dream: the Guantanamo Bay version opens, but there is no danger that Warner's production will sink beneath the weight of right-on didacticism. Few directors are so passionately diligent in their excavation of classical texts that centuries-old emotions suddenly become painfully raw again. Warner's Titus Andronicus in 1987 caused audience members to faint, and one critic described her now legendary first collaboration with Fiona Shaw on Electra as "debilitatingly intense". In 2001, the London Evening Standard paid tribute to the impact of her collaborative work with Shaw when they won Best Director and Best Actress awards for a production that shunned the stereotype of Medea as a screeching, child-murdering witch and reinvented her as a distraught romantic in a black cocktail dress.
Part of Warner's power is that nobody - not even Warner herself - can predict exactly what emotional or physical journey her next production will take them on. In 1995, the St Pancras Project had audiences trooping through the abandoned Midland Grand Hotel on "a fantastical walk" next to St Pancras Station in central London. In 1996, thousands made their way down a backstreet in the East End to find Shaw performing T S Eliot's The Waste Land against the decayed beauty of the - until then - forgotten Wilton's Music Hall. How does she feel about presenting Julius Caesar in what, for her, is now an extremely conventional theatre? "The Barbican's a wonderful space," she says, "and it's giving me the chance to put on a production which will operate on a scale that theatre has been denied for a long time. The moment I was asked to do a Shakespeare, I said that I wanted a hundred extras."
She laughs slightly nervously as she talks about the "overwhelmingly thrilling" power of the actors playing the mob, but she is extremely excited about her main cast, which is led by John Shrapnel as Caesar. Shaw plays Brutus's wife, Portia - "it's a small role, but her placement is heartbreaking and absolutely crucial" - while Simon Russell Beale appears as Cassius, Anton Lesser takes on the role of Brutus, and Ralph Fiennes returns to the stage as Mark Antony. "If you're not lucky enough to get three really good male leads, then you're doomed," Warner pronounces. "This is ultimately a play about the individual, and in the fifth act the psychological exploration is quite extraordinary. It's terribly radical for Shakespeare - in Brutus's role especially, it can feel like you're stumbling on Chekhov."
How does the director who has made her name by laying bare the souls of tragic heroines such as Electra and Hedda Gabler - and who once cast Shaw as Richard II at the National - feel about putting on such an essentially manly play? "Politics is still a very manly world - I don't think we've escaped that," she replies. "And wars are inescapably testosterone-driven, even though women and children are caught up in them." Besides, she says, she's enjoyed tackling "boys' plays" before, even though it worries her hugely that there aren't nearly enough decent roles for women in the existing theatrical canon. Her response to this so far has been to turn to texts that haven't been specifically designed for theatre. "I think The Waste Land was a marvellous part for a woman. And The Powerbook [Warner's adaptation of Jeanette Winterson's novel about cyberspace and romantic love] really gave an idea of how concepts about theatre and writing might be shaken up over the next ten years."
But, for now, Warner is resolutely focused on the work that depicts what she describes as "the most famous murder in the world". She is most transfixed by the cycle of death that Caesar's assassination sparks off, culminating in the suicides of Cassius and Brutus. We do not have to look far in today's politics, she points out, to find numerous examples of suicides and unnecessary deaths resulting from the momentous decision to go to war. "It's not about Iraq," she insists, "but I think that all the cabinet, including Blair, should come and see it before the general election."
Julius Caesar is at the Barbican, London EC2 (0845 120 7518) from 14 April to 14 May