Beneath Lincoln's Inn in Holborn, poor old William Prynne must be turning in his grave. In 1633, the Puritan lawyer wrote a 1,000 page pamphlet called Histriomastix: The Player's Scourge, arguing that cross-gender casting "must need be sinfull, yea, abominable unto Christians" since the Bible expressly forbids transvestism.
Above him, however, London's theatres are wholeheartedly embracing the practice in 2012. Since the beginning of this month, the Apollo has been chock-full of men in frocks, as Tim Carroll's all-male productions of Twelfth Night and Richard III transfer from Shakespeare's Globe with Mark Rylance and Stephen Fry in tow. Later this month, the Donmar Warehouse opens its all-female Julius Caesar starring Dame Harriet Walter and Frances Barber directed by Phyllida Lloyd. Come January, Edward Hall's Propeller Theatre returns for a UK tour of Twelfth Night and The Taming of the Shrew.
All this comes at a time when gender is at the forefront of theatrical politics. In June, the actors' union Equity sent letters to 43 artistic directors calling for increased opportunities for women, after finding that "roles for men significantly outweighed those for women" at the vast majority of theatres surveyed. Only one, the Manchester Royal Exchange, had employed more actresses than actors in 2009/10. With that in mind, can an all-male cast ever be justified?
The practice's roots lie in history. Women being banned from the stage until 1660 when Charles II was restored to the throne, Shakespeare's female characters were played by boys. This is Carroll's rationale for single-sex casting: "trying not to do anything they wouldn't have done, staging it the way they would have staged it with costumes made exactly as they would have been made."
The riposte that usually comes back is that such authenticity is either impossible or reduces theatre to historical re-enactment. Neither argument holds much truck for Carroll: "I've always thought that a glib, lazy argument. We've never claimed authenticity, but whether that's achieved or not, it certainly has the kind of rigour that's always the mark of good work."
In 2002, about to direct the original production of this Twelfth Night, Carroll was "a complete sceptic" about original practices. However, in rehearsals, he found it forced him to question every easy assumption. "It really releases you as a director from the temptation of clichés. As Cesario, Viola often wears an Edwardian sailor's costume. For some reason everyone sees that and thinks it's exactly how it should be, but it's actually bafflingly irrelevant. Changing the period brings white elephants into the process, but you end up trying to clean up elephant poo."
When it comes to equal opportunities, however, Carroll doesn't believe there's a case to answer. "I can see how lopsided classical plays are for men and women. There's no doubt that's a very regrettable thing, particularly galling for all the amazing actresses over 40. But I don't feel the need to justify original practices on those grounds, because [original practice] is a very strong, clearly defined task."
Lloyd agrees, even though her Julius Caesar has sprung directly out of the gender disparity. Approached by the Donmar's new artistic team, Josie Rourke and Kate Pakenham, she initially proposed a play with an equal number of male and female parts. "Then I thought, 'Hell with this, I need to make reparation for some of the imbalances I've subscribed to over the years'."
She's setting Julius Caesar in a contemporary women's prison . "It's less about the opportunity to play the opposite sex and more about getting our hands on the keys to the kingdom," she explains, growing gradually fervent, "It's about access to the ideas and the size of those ideas, seeing what it feels like when we express them. If you're looking for a great play about political disintegration after regime change with great roles for women, where do you start?
"What's exciting is the voracious appetite with which these actors are falling on these roles. They've got the keys to places they've never been allowed and they're gorging themselves on this muscularity and language, this philosophical territory, this amount of violence and taboo."
In both cases, then, the appeal lies in expanding an actor's range into new territories. Just as women rarely get "to be revolutionaries and warriors and dictators", as Lloyd says, male actors playing female characters get an entirely new perspective. "I've always been struck by how eager actors are to play women," says Carroll, "Actors are people who want to experience different realities – what it might be like to murder or seduce someone – and playing a woman involves a big imaginative leap."
It requires more than imagination alone, though. Johnny Flynn, who plays Viola and Lady Anne in the Globe productions, likens the technical requirements "to an athlete's training. You've got to get that vocal range, expressive not sing-songy falsetto, your walk changes and you can't just sit in a chair, because you've got these skirts and a train." Costume, which takes up to an hour to put on, does a lot of the work. "You realise what you can and can't do," he says.
Fresh from drama school, Flynn worked with Propeller, so he's got some experience of single-sex casts. However, the two styles are very different. Propeller are always fiercely contemporary and, crucially, never try to disguise their real gender. "Ed doesn't ask his men playing women to put on a girly voice. They might be in a dress and some lipstick, but you still see bald spots and chest hair." Counter-intuitively, this often has the effect of elevating, not undermining, the female roles. You sit up and listen afresh, rather than focusing on the protagonists.
Lloyd is hoping for a similar effect: "It's really like one of the great orchestras playing a famous piece of music on original instruments. You hear parts of the score you've never heard before. It's not just in the moments that draw attention to the fact they're women, but in the different music of women's voices, bodies and sensibilities."
Carroll agrees that the main benefit is in its very oddity, even its otherworldliness. "So much of what you do is not what an audience expects, even as an Elizabethan production, because most will only have seen a bastardised Elizabethan style. You end up with this wonderful strangeness and unfamiliarity."
'Richard III' and 'Twelfth Night', Apollo Theatre, London W1 (0844 412 4658) to 10 February; 'Julius Caesar', Donmar Warehouse, London WC2 (0844 871 7624) 29 November to 9 February