Before he was Gandalf and Magneto and she was a succession of purse-lipped English queens, Ian McKellen and Judy Dench were most famous for being the Macbeths.

Acting Shakespeare is like nothing else an actor is called upon to do. If you're playing Magneto you've got something like a clear run at the part. No-one has done it before you. But if you're playing the Thane, you're speaking words that every theatrical hero for the past four hundred years has rolled round his mouth.

So how on earth do you make it fresh? Watching the McKellen/Dench Macbeth yesterday (it was a stage production for the RSC which Thames TV- bless them-undertook to film ) it took me an act or two to get over the familiarity of a text in which almost every phrase has become a proverb or a cliche, and experience it as drama rather than recitation.

That I got there eventually is thanks largely to director Trevor Nunn's insistence that every single word should make human sense. I've noticed this before with his work. He has a passion for the meaning of Shakespeare's text. Everything that is said is made to serve character and relationship. This is true even of the sing-song of the witches. Instead of hurrying through the gobbledy-gook, Nunn has taken it apart, polished and cleaned every cog and spring and then reassembled it so that instead of a halloween farago we now have three sharply distinguished characters- two older women and a young one who serves them as a trance medium- in what looks like a real and well-practised working relationship.

In a production like this there can be no spear-carriers. Attendant Lords have attitude and the stars emerge as first among equals. The role of Macbeth is an alp that has defeated a lot of high climbers. Compared to Hamlet or Lear it's an underwritten role. The character falls from grace and into disaster in very few lines. If Mckellen succeeds it's because of Nunn's policy of making every word count. His Macbeth is a lumpy faced boy with sleeked-back hair, very ready with the false smiles. The banquet scene, played without a visible ghost, is the production's climax. McKellen shambles and roars and drools.

Where McKellen is fitful, Dench is steady. She can take more pressure than he can, but when she breaks she breaks suddenly. She shatters. He is the storm and she is the moonlight. Her face- that strange square face- is like a skull.

The play is done with few props on a bare stage, and in close-up against darkness. This, I swear, is the only way to perform Shakespeare on TV. The one thing that distracts is the curious mix of medieval and Victorian costume- of frock-coats and armour. If the men have watch-chains, why don't they also have guns?

Shakespeare is political. He writes about power. About how human nature copes with its allure and weight. Macbeth is his most extreme exploration of the theme. There are saints (the off-stage Edward the Confessor) and there are monsters. But what is so terrifying about it is not the apparatus of ghosts and witches but the revelation of how little it takes to send a brave and decent man down the fun-house shute into a delirium of violence and paranoia. It is easy to turn it into a gothic romp, but this production, with its refusal to waste a line, never lets us forget that these are people- real people- going to the bad. And as McKellen and Dench, faces close to ours, speak their characters' hopes and doubts and madness, we are compelled to identify with them. There, but for the grace of God, go I- if only (of course)I were big enough.

No production of any Shakespearean play is ever definitive. But this comes close. It's certainly the best Macbeth available on tape or DVD.