On the West End stage tonight you can see Mark Rylance playing Olivia in an all-male Twelfth Night. Shortly you will be able to see Harriet Walter playing Brutus in an all-female Julius Caesar. And reports this week suggest that on screen you will next year see a female Captain Mainwaring in Mum's Army or, at any rate, a radical remake of Dad's Army.

As the feature on page 22 of Radar makes clear, these are heady times for cross-gender casting on stage. Gender-blind casting is, it seems, catching up with colour-blind casting. Having seen this production of Twelfth Night when it was at Shakespeare's Globe I can confirm that the all-male cast, as in Shakespeare's day, brings a new (or rather four centuries old) dimension to the play, a whole new level to the humour and the comic lines.

Once, actresses might have felt a bit resentful that they were losing out on great Shakespearean roles here, but the all-female Julius Caesar at the Donmar should ease that resentment considerably. It will be interesting to see if that brings any new insights to the play, and indeed whether other all-female productions of other classics will follow it.

So far so good. But can one have too much of a good thing? Can a spirit of radicalism and textual exploration become a gimmick which renders a text and an idea meaningless? That's my feeling with the proposed film remake of Dad's Army featuring a female Captain Mainwaring. This paper suggested this week that the likes of Sue Perkins or Prunella Scales could play the role.

Of course, one can have a pompous, social climber who is a woman just as much as a man. And there are more than enough actresses who could play the role well. But while there might be plenty of laughs, the historical and social context is lost. Though there were some women in the Home Guard, there were no female platoon commanders, and the history is too recent for the viewers to ignore that fact. A comedy that was so convincing because of its characterisation in a social and historical context would become a fantasy.

What a pity the producers don't consider a series on Mrs Mainwaring and the other women we hardly saw in the original. That could be both funny and illuminating.

I worry about this because I suspect it is with TV classics, rather than on stage, that we will see an ill-thought out rush to cash in on cross-gender casting. In some cases it could work with great originality. I could easily envisage a remake of Steptoe and Son, with mother and daughter replacing father and son. But most classic sitcoms would lose the very basis of their humour – social realism.

Shakespeare's characters have an almost mythical status. One can take liberties with them and still relish the language, and enjoy a different texture given by cross-gender casting. Twentieth-century sitcoms are tightly rooted in time, place and the pervading social order. In a funny way, they are a lot less flexible than Shakespeare is.

Disharmony in the Mahler camp

Conductor slams composer. And his fans. All too rarely is the musician/composer reverence and deference challenged. So I find it refreshing that Bernard Haitink has taken a poke at Mahler and his aficionados in the European classical-music magazine Das Orchester. "It's a hobby-horse of mine and a major worry, this Mahler cult," says Haitink. "There are people who come to a concert only if Mahler is played. Once, after a performance of Mahler's Third Symphony, I received a letter, telling me: 'I was so moved, I wept through the whole piece.'" I almost wrote back: 'You need to see a psychiatrist.'" He should have done. But remind me, who brought out a 10-CD compilation of the complete Mahler symphonies? Bernard... Bernard... it's on the tip of my tongue.

Turning the ROH into a student bar

Full marks to the Royal Opera House, which I hear is appointing 21 student ambassadors to plug the Royal Opera and Royal Ballet on campuses across the country, particularly their £10 student standby tickets, with, on certain evenings, the entire amphitheatre given over to students. A Royal Opera House spokeswoman assures me that, on student nights, the bars will have a more student fare – beer and crisps replacing champagne and smoked-salmon sandwiches. I suspect that might be to underestimate the tastes of today's students. Scrap the beer and crisps. Just drastically reduce the price of the champagne and the smoked salmon – and, come to think of it, for us non-students too.