An article by Matt Trueman in the Time Out theatre section this week, on how stand-up comedians are turning themselves into actors, refers to the reverse procedure, of classical actors proving their worth in comedy "in order to be thought truly great."

It's a strangely tortured argument, especially as the example given of the latter phenomenon, Paul Scofield, was always a brilliant comic actor and had nothing to prove about his greatness even by the time he played King Lear. Trueman says that Scofield spent most of 1958 in "a ridiculous comedy" called Expresso Bongo.

In fact, Expresso Bongo was a trail-blazing satirical musical by Wolf Mankowitz, Julian More, David Heneker and Monty Norman in which Scofield played an unscrupulous music producer who discovers and promotes the first new British rock and roll singer (the show was based on the Tommy Steele story). It was the Look Back In Anger of British musicals and paved the way for Joan Littlewood and Lionel Bart, not to mention Stiles and Drewe in their present contribution to the Cockney bohemian genre, Soho Cinders.

The idea that Scofield was somehow slumming in comedy, or even musicals, is in itself deeply patronising. He was indeed the greatest King Lear I've ever seen. But he also gave monumental, unmatched performances as Khlestakhov in The Government Inspector and as a gay hairdresser in Charles Dyer's Staircase; he was a past master at out-of-control vanity and affronted prickly grandeur.

The odd thing is that he failed in so many great Shakespearean roles - Macbeth, Prospero, Othello - and only really rescued his high-end classical reputation in Richard Eyre's great National Theatre revival of John Gabriel Borkman.

Ibsen defined Maggie Smith, too, in that way: she was superb as Hilde Wangel and Hedda Gabler, but her greatness was rooted in her comic performances, just as it is with Sheridan Smith, also playing Hedda any moment now.

As for stand-ups proving themselves as actors: it doesn't really sound as though Marcus Brigstocke pulled it off in Spamalot. Catherine Tate certainly didn't as Beatrice (Eve Best showed how it should be done at the Globe in the same season). Nor did Omid Djalili in What the Butler Saw. And if Rob Brydon improves on Colin Blakely in Alan Ayckbourn's A Chorus of Disapproval, or Stephen Fry on, say, Nicol Williamson or Donald Sinden, as Malvolio in Twelfth Night, I shall eat my toes. Great actors always do comedy better than comedians.

Why? Because they know about breathing, timing that goes beyond mere rat-a-tat back chat, physical movement and the architecture of a performance as opposed to its mere and easy discharge. And their voices are musical instruments, fully trained and maintained at concert pitch. Anyone can play Hamlet. But very few play him well.

Another thing: great acting is about mystery as well as magic. What great secrets were kept by Scofield, or by Ralph Richardson, or indeed nowadays by Vanessa Redgrave or Judi Dench? Who will plumb the depths of Rob Brydon? Who will want to?

The death of Gore Vidal reminded me that, had he lived, Kenneth Tynan would have been the same age, 85, right now. He died aged 53 in 1980. What would he have made, I wonder, of the changes in criticism in the internet revolution, or the odd statement the other day by Mark Shenton that blogging was the most important part of his work as a journalist (making a distinction between journalism and criticism)?

Blogging obviously incorporates critical writing - well, mine does - but it's not theatre criticism. It's writing about the theatre and/or related issues. Tynan's "blogging," I suppose can be found in his essays which - like those of Gore Vidal and indeed the recently late Christopher Hitchens - are among the best in our language.

So we've lost Vidal, Hitchens and now Robert Hughes, the best of all art critics and another peerless essayist. I read blogs because I enjoy them, and they're fun. But for proper, well-written criticism I read Michael Billington, Susannah Clapp, Philip French, A A Gill and Clive James - though not James in his latest incarnation as TV critic on the Saturday Telegraph, a pale echo of his glory days on The Observer.

James, alas, is fading fast, too, stricken with a rare form of lukaemia, the details of which he has movingly documented, but only when the news got out. He keeps his private life to himself. Unfortunately for him, there are also complications in that private life, gloatingly recounted in yesterday's Daily Mail, a newspaper that has no qualms about kicking people when they're down. I suppose that counts as criticism these days in Derry Street.