Antony Sher's performance is literally towering at the opening of the play, directed by his husband, RSC Artistic Director Gregory Doran. Lear is borne in on a huge platform above the glittering monochrome of his court (designer Niki Turner), wrapped in fur cloaks that make him larger than life, his every pronouncement accompanied by thunderous chords to make of him a primitive demigod. He may look "every inch a king", as he says ironically later in the strange lucidity of his madness on the cliffs at Dover, but he is equally a very foolish old man, as he also refers to himself later. His rejection of youngest daughter Cordelia (Natalie Simpson, all quiet resolution in white) is especially cruel, arbitrary and yes, senile, simply because of that god-like build up.

But it is the reaction of oldest daughter Goneril (excellent Nia Gwynne, an auburn-plaited Saxon princess in russet jewel-encrusted gown) that is most startling. Foreboding at the impropriety of his asking his daughters how much they love him turns to horror on Goneril's face, as her father turns the full force of his cruel rage on Cordelia for her honest reply. Goneril’s fears are well-founded of course, for later he curses her womb, and the physicality of Sher's spite as he grabs hold of her in a cruel travesty of an embrace and her momentary hopeful and needy response to it are all the more shocking.

Doran also gives Lear several of the hundred knights demanded to keep for his retinue to carouse with him around his daughter's table, and a downright noisy boorish shower they are too, so that to start with it's hard not to sympathise even with Kelly Williams's vivid scheming middle daughter Regan.

This engaging of sympathy for a child who will ultimately prove unnaturally cruel is echoed in the relationship between David Troughton's exceptional Gloucester and his bastard son Edmund (Paapa Essediu, a villain with a fine sense of irony), clearly nursing a 'legitimate' grievance as his father introduces him to Kent with that well-worn tactless joke about the "sport" he had conceiving him.

The brilliance of both Sher and Troughton is in their ability to engage sympathy once they are changed by what they endure. Sher sloughs off the layers of clothing that make him imposing from the outside, as he gradually gets to know himself and understand reality and, for the first time, other people. If he is touching in his madness on those cliffs, it's because he is content – even happy – in that altered state (in the way that dementia patients often present for example). The audience learns to love him as he learns himself and is truly reunited with Cordelia.

The parallel reunion between Gloucester, who only sees clearly once he has lost his sight, and his true loving son Edgar, forced to disguise himself as a mad beggar when Edmund convinces his father he’s the villain, is equally moving thanks to Oliver Johnstone's resourceful Edgar, proving ultimate filial devotion as his father, like Lear, achieves closure at life's end.

By Judi Herman

King Lear runs until Saturday 15 October, 7.15pm & 1.30pm, £16-£70, at the Royal Shakespeare Theatre, Stratford-upon-Avon, CV37 6BB; 01789 403493. Then at the Barbican Theatre, London, ECY2Y 8DS; 020 7638 889, 10 November to 23 December, 7.15pm & 1.30pm £25-£55 . In cinemas from 12 October.