Peter Brook told me recently in Paris that there's no such thing as the avant-garde anymore. What else could possibly be new in the theatre? Anything innovative seems to be technical rather than experimental. We live in different times. I don't think these were merely the ruminations of a very old man.

And yesterday in a Daily Telegraph interview, Simon McBurney, who is preparing one of the most awaited new shows of the year, his staging of Bulgakov's master work, The Master and Margarita, at the Barbican en route to the Avignon Festival, said that, "there's nothing new in the theatre, ever."

The point about this is that people who work in theatre have to believe that there is. Thus the excitement surrounding Punch Drunk, or DV8's new piece about Islamism and censorship, or even London Road at the National last year.

Unless each generation believes that, say, Jerry Springer: The Opera is a sensational new breakthrough (it wasn't) then they might as well pack up and go home. It's the critic's job to go with that sense of excitement but also to place it all, where possible, in historic context.

Still, it's hard not to admit that the work of, say, Sound&Fury, those redoubtable sculptors of light and sound, bringing their new piece, Going Dark, to the Young Vic next week, are not in some way pioneering a new, or new-ish, form of theatre.

Similarly, McBurney himself has taken his company Complicite to the tipping point between art and science in both Mnemonic, an intensely difficult piece about memory and brain functions, and the more "accessible" A Disappearing Number, which mixed a biographical detective story with a foray into pure mathematics.

Where McBurney is totally correct is in his dismissal of the terms "immersive" and "site-specific" as having any meaningful cache in the critical theatrical discourse. Audiences have been immersed in theatre since the day it started, and more esoteric devotees have been trudging round hotels, disused factories, quarries, sea-shores and mountaintops for at least 50 years.

"Theatre-makers are thieves, in the honourable tradition of charlatans," says McBurney, "they fake it very, very well indeed for the entertainment of everybody else." That's the spirit.

It was always said of Peter Brook, in his early years, and often by his own collaborators, that he was a magpie, an intellectual synthesiser of pure theorists like Brecht, Artaud and Meyerhold. And that, of course, was his genius. You see the same sort of process in the very best work of Rupert Goold, or Richard Jones, with added cultural accretions from music and modern art.

All good theatre, like all good music, absorbs what happened before. Even last night's surprise package at the Trafalgar Studios, The Leisure Society, starring Ed Stoppard and supermodel Agyness Deyn, feels one minute like an Edward Albee play, the next a Yasmina Reza. And it actually derives from French-speaking Montreal.

Similarly, Zach Braff's All New People at the Duke of York's, which one or two critics have got very steamed up about, dares to reinvent the notion of theatrical backstory using startling, overblown video footage ("What happened to exposition?" squeaked Michael Billington, no doubt with Ibsen in mind, as usual) in a comedy that is intensely interesting at the very least in its cultural information and in its explanation, through dialogue, as to why all four characters have arrived in this god-forsaken place.     

All New People is also a very funny script. The play is much better written than Ishy Din's Snookered at the Bush, which has rightly attracted jolly decent reviews, but which deteriorates as it goes on, partly along with the characters' boozed-up exclamations.

Not for the first time you're left wondering at London theatregoers' (and critics') great good fortune in having three or four brand new plays in the space of one week to have a good old argy-bargy over. That's the real strength of our theatre; and the actors.

Ed Stoppard has certainly surprised me with his comic timing and demeanour in The Leisure Society. And Agyness Deyn could easily become a slinky new fixture in modern comedy and, who knows, Michael B's beloved Ibsen.

Her arrival was certainly noted by the theatrical paps: crowding into the tiny second Trafalgar Studio at the curtain call last night was not only Whatsonstage.com's own Dan Wooller, but also man-about-town Dave Bennett and - long time no see on this circuit - the shutter king himself, Richard Young. Check out those after party snaps very soon.