As I watched the National Theatre’s riveting production of Othello a couple of weekends ago, I thought how interesting it would be if Adrian Lester, in the title role, and Rory Kinnear, the actor playing the longer part of Iago, were to be given a chance to swap over some time during the run. This would revive a long tradition of such exchanges in this Shakespeare tragedy. William Charles Macready and Samuel Phelps famously did it at Drury Lane in 1837. Richard Burton and John Neville followed their example at the Old Vic in 1955.

Some might perceive an immediate objection to the move in that Mr Kinnear — a fine actor whose prowess was first shown on the student stage here in Oxford — would need to ‘black up’. This is something that no actor has really felt able to do since the 1960s when Laurence Olivier became the last high-profile white actor as Othello.

But is this necessarily the case? Audiences have long been used to ‘colour blind’ casting and see no problem with, for instance, a black actor playing the part of someone known to be white. Mr Lester, as it happens, supplied an illustration of this ten years ago in his acclaimed portrayal of Henry V for director Nicholas Hytner at the National Theatre.

I think it extremely unlikely that Sir Nicholas considered, even for a moment, casting a white actor as his new Othello. For some reason this remains utterly unthinkable. But why should this be so? Why is it that — almost uniquely — Othello is a character that must be seen to be black and, that being the case, must necessarily be played by a black actor?

Ah, someone will say, his blackness is integral to the plot. But is it? Not really. True, there are a number of disparaging references to it, principally by Iago and Desdemona’s bigoted father, Brabantio, but Othello’s colour is not a big part of the story in the way, say, that Shylock’s Jewishness is in The Merchant of Venice. No one would insist, surely, that only a Jew could play the money-lender.

That theatre audiences are not always shown what the script says they are seeing is neatly illustrated in two plays being aired at present on the London stage.

In the first, Tennessee Williams’s Sweet Bird of Youth at the Old Vic, both of the main characters are markedly easier on the eye than they are supposed to be. Kim Cattrall is too drop-dead gorgeous to be convincing as the ‘fading’ movie star Alexandra Del Lago; ditto Seth Numrich, as her much younger companion. Chance Wayne is precisely said to be 29 and close to the end of the road in his trade of gigolo. In the words of a reviewer with The Huffington Post he is “frazzled”. Seeing Seth, aged 26, in the role you might think the adjective ‘ripped’ more appropriate. For much of the first act, he is stripped to the waist, and indeed beyond it, looking in his skimpy drawers just like a model in a Dolce & Gabbana advert.

Then there is Daniel Radcliffe as The Cripple of Inishmaan in Martin McDonagh’s play of the same name at the Noël Coward Theatre. Quentin Letts put the matter neatly in the Daily Mail: “He has never been kissed and has eyes reportedly as ugly as a goat (that last quality is a little elusive, because there is no disguising Mr Radcliffe’s good looks).”

The curious thing about this ‘only black actors for Othello’ rule it that it is being clung to at a time when we are encouraged elsewhere to be not only colour blind but also accent deaf. This was famously seen, for instance, in the Royal Shakespeare Company’s Hamlet of 2008 with David Tennant as a Danish prince with a distinct Scottish burr to his voice. Confusing, eh?