Thomas Sutcliffe: Here's hoping sparks will fly

It was easily the double take of the year - a momentary hesitation by Judi Dench as she made her entrance at the back of Stephen Brimson Lewis's set for the Royal Shakespeare Company's new musical version of The Merry Wives of Windsor. His design includes an optical trick - the downstage houses looming life-size while those at the rear are just bigger than dolls' houses, a perspectival device which is fine just so long as no one actually stands next to them. Obliged to enter between two of these Lilliputian buildings Dench caught herself momentarily and gave the roof tiles a puzzled stare - as if astounded to find the world dwindled around her. It brought the house down on the night I saw the production - and if you'd been looking somewhere else for that moment, down at the programme for instance, no double take would have recovered it for you, it was so fleeting.

Double takes are hardly the most sophisticated or subtle of an actor's effects - but it was very beautifully done even so - level and admonitory and managing to convey a sense that it was borderline impertinence in these buildings not to be a good deal larger. And though I've never been one of Dench's biggest fans (indeed although I suffer from a mild allergy to her presence in a film or a play) it made me see - for a flash - what so many other people see in her. There was a command at that moment - both of the exact physical gesture and of the audience watching it - that was deeply reassuring. That's why, I thought, you so often hear that little sigh of contentment from the stalls when she makes her first entrance.

I'd experienced something similar in New York recently, at the opening of David Hare's The Vertical Hour - watching, or rather feeling, a Broadway audience getting to know Bill Nighy. English audience members had already arrived at the point of contented familiarity, so when he climbed up from below stage with that soigné slouch we knew roughly what was in store for us. Those who hadn't seen him before had to learn to appreciate him - and everything suggests that they enjoyed their lesson in the Nighy method - that strange mixture of twitchiness and languor. Every now and then - on a particular line or look - you could sense their pleasure cresting for a moment.

What was notable in both cases, though, was the sense that a good performance always rests on fugitive instants. A performance might be conceived as an organic whole - flowing with architectural calculation from one kind of tension to another, through-composed by the actor so that the themes and variations all melds but it's likely to be received by an audience as a series of dazzling impressions. This is partly a trick of perception itself - thinking back over a good performance we naturally highlight those bits that will best pin down its excellence and forget the overall texture that sustains them. We remember in snapshots, rather than in long video clips. But it's also because acting is an art which depends on unsustainable effects.

Watching Dench I was reminded of Coleridge's famous remark about Edmund Kean, the effect of whose acting the poet described as like "reading Shakespeare by flashes of lightning" (even though in this instance Shakespeare remained blankly unilluminated). I'd always taken this to be a remark about the intensity of Kean's light - that what was previously obscure was suddenly brilliantly lit up. But it also catches something of now-it's-here, now-it's-gone quality of a great performance - the way that it reveals itself in instants that seem viewed under a different light than most of the play. It's a paradox that means great acting is often recalled as a series of apparently trivial details.

Of course it isn't impossible - after the storm is over - to piece together a more coherent account of the weather, and to admire its larger movements. And simply in practical terms we know that actors have to be able to predict where lightning will strike - once a night, at least, and twice on days with a matinee. Coleridge's simile may imply that Kean was merely the lightning rod for a greater power - but the implication is probably off beam.

A good actor's effects have to be, to a degree, predictable and repeatable. If a performance actually felt like that as the audience experienced it, though, how could it possibly be described as great? The startled reflex, whether expressed in laughter or by sudden stab of unexpected emotion, is an essential part of the theatrical experience. We might politely pay attention to actors for two or three hours at a stretch - but the moments which make the politeness worthwhile are when a spark suddenly flies between performer and audience.