The tradition that performances are punctuated by the rise and fall of a sheet of cloth survives in everyday language. We talk about the "curtain coming down" or "curtain speeches" being made, particularly in politics. When a career ends (Ken Livingstone, Nicolas Sarkozy), we often just simply say that it's curtains.

The irony is that, in theatre itself, curtains are becoming an endangered species. Most newer theatres – especially studio spaces or buildings with thrust stages – don't actually possess one, and even in more traditional spaces directors seem to be using them less and less.

Audiences arriving at London's Royal Court's upstairs space for Bola Agbaje's excellent new play Belong – in which a black British politician takes a sabbatical in his Nigerian homeland after the curtain comes down on his career – find the actor Lucian Msamati, who plays the defeated MP, beached under blankets on a sofa in the middle of the stage.

And in the recent Richard II at the Donmar Warehouse, Eddie Redmayne sat impassively on a throne in the middle of the set, staring directly at incoming theatre-goers. I remember many years ago another political play, Doug Lucie's Progress at the RSC, which also offered early arrivals the sight of actor Brian Cox comatose on a sofa.

The main reason for these pre-textual prologues, which blur the start time advertised in the papers, is presumably the pursuit of greater realism: a sense that the performance is joining a story that has begun some time before, rather than requiring a sudden suspension of disbelief.

However, the strategy is risky because it creates unease in an auditorium. Caught in a threshold territory where we are not really "in" and the actor is not strictly "on", some play-goers – and this is a device that applies only to the timely section of the audience – start whispering or sit in silence, rather than participating in the banter and catch-up that are part of a theatrical evening. Perhaps worse, though, are those ticket-holders who carry on as if the actor isn't there: on the night I saw Richard II, a couple had a loud and uncomplimentary discussion about Redmayne's performance in the film My Week With Marilyn, while he sat only a couple of feet away. Actors required to take part in such pre-scenes presumably need skin (or ear wax) of considerable thickness.

Another weakness of the technique is that, from courtesy to the audience and the bar staff, there still has to be a moment when the performance formally begins, with lights going down in the auditorium, if not on stage. So the moment of transition from pre-play to play is merely more gradual rather than removed. The tactic would also obviously be inappropriate for those many modern plays – especially by David Mamet or Neil LaBute – in which the first scene begins in mid-speech or -argument: the characters cannot stand frozen on the stage until everyone is in.

If the line has to be blurred, my preference is for the use of musical performances that preview the mood of the performance without paralysing the audience into embarrassment, as achieved by the skiffle group before One Man, Two Guvnors and a string band that entertained audiences before another recent Donmar show, The Recruiting Officer.

But such build-up tricks should be used carefully and sparingly. The audience understand that the characters they see on stage are someone else and somewhere else immediately before and after. Many productions punish theatre-goers who arrive late by refusing admission completely or until a scene change. So it's only fair that anything done to those who turn up early should be genuinely rewarding.