Quentin Letts: Never mind the play, just listen to the audience

During a pause in 'Pygmalion', a respectable male voice shouts: 'Go on, give her a snog'

When a stage performer is enacting the anguish of an abortion the last thing she expects is audience laughter. That was the sound which greeted Kathryn Hunter at London's Soho Theatre last week – persistent, hooting laughter from a group of wrigglers in the second row. It was almost enough to make you sympathise with the recent declaration of another actor, Ian Hart, who had a Gerald Ratner moment and told an interviewer, "I hate the audience".

Miss Hunter kept going with her abortion scene in The Diver, a Westernised version of Japanese Noh theatre. The ill-timed laughter came from some children, aged 11 or 12, who should never have been taken to such a difficult show in the first place.

This was but the latest in a series of incidents which suggest that London theatre audiences have forgotten how to behave. In addition to old failings such as ice cream tub scraping and the rustling of sweet wrappers, there are now problems not only with mobile telephones but also with palm-held computers. Bladders seem never to have been weaker, late arrivals clamber over everyone else to get to their seats and traditional English reserve has given way to Mediterranean levels of chatter.

Ruby Wax, who should know better, whipped out her Blackberry to check emails while watching (the excellent) Much Ado About Nothing at the Royal National Theatre. Confronted afterwards about her bad behaviour by novelist Deborah Moggach, an unfazed Wax shrugged. She said she didn't think anyone had been able to see the bright glow of her machine's screen.

The Stage newspaper's Mark Shenton encountered similar insouciance from a theatre professional at the Donmar Warehouse. During Uncle Vanya a man near Shenton kept receiving text messages, each causing a loud "bleep". "We complained to him afterwards," says Shenton. "The guy said: 'Oh, I think Chekhov's robust enough to cope with that, don't you?'." The offender was Michael Colgan, director of Dublin's Gate Theatre.

In my own peregrinations as the Daily Mail's theatre critic I have seen people squinting at newspaper crosswords, opening crisp packets, eating homemade sandwiches and taking photographs of each other with their mobiles during performances.

In the middle of May I was at the Old Vic, enjoying the last preview of Pygmalion, when we, or rather the relationship between Henry Higgins and Eliza Doolittle, reached a crucial moment. Dramatic pause. Higgins, speechless, stares into Eliza's dreamy eyes. Total hush. Then, from the middle of the house, a respectable male voice: "Go on, give her a snog!"

During Joseph And The Amazing Technicolor Dreamcoat there were wolf whistles and shouts of "get yer loin cloth off" when Lee Mead came on stage. Hunky Lee landed the part after a TV talent show and is now regarded as semi-public property. So be it. But Sir Ian McKellen's decision to strip naked when playing King Lear for the RSC has also been prey to ribaldry. The night a friend of mine saw the production, Sir Ian's pendulous nudity earned a yelp of "good GOD!" This is not what you need towards the end of a Shakespearean tragedy.

Perhaps it is time audiences were instructed in theatre etiquette. At the National's Cottesloe auditorium a few weeks ago, security guards had to be called in the interval to eject unruly teenagers who were thumping each other, talking and generally acting the goat. What a pity they weren't lined up beforehand by a senior usher and told firmly how they were expected to behave. Actors have started to take matters into their own hands. The actor Richard Griffiths has started a sort of stage jihad against members of the public who fail to turn off their mobiles. We may need more of this.

The troublemakers do not always intend to be difficult. Random, by Debbie Tucker Green, is a monologue about the murder of a black teenage boy. The first night Royal Court crowd included many theatre virgins who kept whooping and slapping their knees, even though the story was profoundly sad. They didn't mean to be disrespectful. I think, in fact, that they were trying to do the right thing and show support. A quiet word of advice would have put them right.

Unpredictability is, however, one of the charms of theatre. Performances change from night to night and audience behaviour is a legitimate part of the experience. The Royal Court recently staged The City. Its author, Martin Crimp, has been hailed by some as a genius but personally I can't see it. Nor could one member of the audience the week after it opened. While the actors were taking their curtain call he leapt up and shouted: "That was terrible." This led to a furious debate among nearby theatregoers – even while the actors were still taking their bows on stage. Some punters tutted and told the man to sit down. Others said that he was quite right.

The row continued outside in Sloane Square. Terrific stuff.