Slightly surreal, this one-off yesterday afternoon: Helen Mirren gave an audience to a live audience on the Gielgud Theatre stage where she's appearing (until Saturday) in The Audience.

Helen Mirren at the 2013 Olivier Awards
Helen Mirren at the 2013 Olivier Awards
© Dan Wooller
Why would she bother? Because it was a fundraiser for the National Youth Theatre, where she first made her name as an incredibly voluptuous 16 year-old Cleopatra - she had Harold Hobson of the Sunday Times reaching for his superlatives - and she knows they do a great job.

Her onstage interlocutor was Paul Roseby, the third artistic director of the NYT in succession to founding legend Michael Croft - a schoolmaster who carried his passion for Shakespeare into the holidays with sixth formers from all over the country - and Ed Wilson, who kept the pot boiling.

Mirren said that, like so many people, she owed her career to an enthusiastic English teacher at her school in Westcliff-on-Sea, Essex, who told her about the NYT. She auditioned and was cast as Cleo - but, she said, Michael Croft was completely in love with moving crowds of boys around all day as the various armies - and she was pretty much left to her own devices.

Her first job, before embarking on the great learning curve of the RSC, was in a play directed by Braham Murray at the Sunderland Empire, in front of tiny audiences who became tinier (and noisier as they stomped out of their seats) every time she said the line, "Will you shaft me?" Where was that line from? She couldn't remember.

Nor could Paul Roseby. He threw the question open but both Dominic Cavendish of the Telegraph and I were too shy to shout out Little Malcolm and His Struggle Against the Eunuchs. We were, after all, surreptitious guests, not funders, nor participants, nor NYT members.

Microphoned, and saving her voice for the evening performance, Mirren positively radiated a wicked charm in talking about things she must have reiterated countless times: the trip to Africa with Peter Brook - in search of a theatre beyond meaning, but where they discovered that the two things that always work in the theatre are sex and violence; the importance of mentors (generous Ian Richardson at the RSC was one of hers); the joy of finding the role of Jane Tennison in Prime Suspect on television.

Her producer on that show, Andy Harries, also produced her movie, The Queen, precursor of The Audience, and he greeted her from the dress circle. "My God, I thought you were having lunch with my husband..." she cried (husband is film producer Taylor Hackford). "So, where's your husband, then?" enquired Roseby, somewhat cheekily.

She explained that she hadn't wanted to reprise the monarch on stage but that when Peter Morgan sent her the script, she thought, "You bastard." And when she reluctantly attended the first read-through, and saw the production team, all of whom she admired, she just had to do it. She agreed that her real breakthrough had been The Long Good Friday, but she had to have her role re-written by Barrie Keeffe because it was "a bit insipid."

"I've often re-written things. When I'm sent a script I always read the last page first to see if the character is still there. If she's not, then I go back to her last scene, and if it's a great one, then OK, I'll go ahead..."

Roseby had lined up a few of his NYT summer members to ask questions, and the future Juliet was treated to an impromptu recital and analysis of "Gallop apace ye fiery-footed steeds." How did she keep up her energy levels? She quoted Edith Evans saying that you need a strong voice and a strong body. And she (not Edith) particularly recommended yoga to strengthen the spine and maintain limberness and suppleness on the stage.

Two other pieces of advice completed a marvellous, practical masterclass: "Don't be up your own bum... and pick up your cues: think faster, and if you can't think, just do it." Good directors, she added darkly, had to be fully prepared, but not tied up in concepts, and had to have a sense of staging but, most importantly of all, a sense of rhythm; "Peter Brook has all of those qualities in abundance, which is why he's such a great director."

The informality of my day continued across town at the Royal Court, where Vicky Featherstone's Open Court season launched with the first "Surprise" performance by Mark Ravenhill. This was a strange 50-minute lecture of weakly provocative, disconnected statements, delivered at a lectern and revealed, at the end, when programmes were distributed, to be a collation of comments edited by Ravenhill from conversations with other playwrights answering the question, "What is your ideal theatre?"

"Part of me wants to firebomb the Royal Court," said one. "Close down the subsidised theatre," said another. Yet another excoriated all this outreach and education "bollocks." My, what daring. Careful what you wish for, is what I say. It was all too much for a few folk in the front rows who marched out, two of them right across the little stage. Ravenhill didn't react, just ploughed on, quaintly and quietly regardless.

Cakes and Finance, as the piece was called, was a mock manifesto with a yearning for a popularity and a back-to-basics in theatre that sounded faintly ridiculous, and not much of an advert for the wit and wisdom of - can you believe it? - April de Angelis, Howard Brenton, Tim Crouch, Chris Goode, Zinnie Harris, Ella Hickson, Gregory Motton, Philip Ridley and Simon Stephens.

Much better was the Lost in Theatre programme of hidden plays which you can seek out around the theatre at any time of day by borrowing a headset, or uploading from iTunes onto your personal MP3 player, or downloading free from royalcourttheatre.com/lostintheatre.

As a guinea pig for the project, the first ever customer, I'd recommend the headset-on-site approach, as each short piece (ranging from five to twelve minutes) is written for a specific location: on the balcony overlooking Sloane Square, by the red curved wall above the bar, in the alcove beneath the sponsors' roll call en route to the Theatre Upstairs, in a backstage shower (with a stage-manager banging on the door).

The authors include E V Crowe, Abi Morgan, Clint Dyer, Archie Maddocks and Leo Butler, and they all paradoxically prove that writing for theatre probably prepares you better than any other training for writing for radio.