Passion, Donmar Warehouse, London<br/>Krapp's Last Tape, Duchess Theatre, London

Does beauty make us love, or does love make us beautiful?

Artists like this conundrum, and it's one that suits musical theatre, with its flair for transformation. Passion, Stephen Sondheim's 1996 adaptation of a 19th-century Italian novel, came after Phantom of the Opera and Beauty and the Beast, but finds common characters – the repellent creature starved of love and the reluctant object of its affection.

Passion opens with a scene of delirously abandoned sex in a Milan love-nest where fleshy frescoed couples writhe seductively across the walls. Handsome Giorgio and delectable Clara are all beauty, revelling in their marvellous bodies and exquisite bliss, but the happiness they sing of so exultantly crashes in an instant with news of the soldier's imminent departure to barracks far away.

From now on, this heady affair must be conducted by letter, and as elegant Giorgio messes in dutifully with coarser officers it is only Clara he sees and hears – and so do we, as Scarlett Strallen steps through these raucous scenes, seemingly borne through the air by Neil Austin's miraculous lighting and her own luminescence.

The soldiers shrug at hysterical screams off-stage – their colonel was raised in the family of his pitiful and damaged cousin, Fosca, for whom he now cares. When she finally appears at table, for she does not usually eat, it is a ghastly shell of a woman, emaciated, pallid, shrunken and grotesque, who beholds the fetching new arrival with beady-eyed ardour. Here is a romantic hero to equal those of the novels through which Fosca lives vicariously, and her shameless campaign to win this generous-hearted gallant is unleashed, to his dismay, and ours.

The bored officers are delighted by this unexpected sideshow, and here the ensemble work of director Jamie Lloyd's actors is outstanding – they are Chorus, minor characters in a flashback, and change each scene at the double, so that the piece, played without an interval, careers unchecked to its conclusion, propelled by Sondheim's score and urgent lyrics and Alan Williams's brisk musical direction.

With Allan Corduner twinkly as the well-intentioned mess doctor and David Thaxton in good voice as Giorgio, it is Elena Roger who turns in a performance of astonishing virtuosity as the dwarfish and transfigured Fosca. And Sondheim's answer to the looks/love riddle is filed under Everything Comes at a Price: "Beauty," hisses Fosca, "is something one pays for."

Michael Gambon, slumped over the side of a funereal desk at the start of Krapp's Last Tape, has given up on beauty – and just about everything else. He is a corpse-like husk, slowly reviving himself with tugs and taps until he creaks to his feet for an investigative shuffle round this wooden tomb. With his immobile features and long questing fingers he is like any of the higher primates, and lo, there is business, sometimes lewd, with a banana. Chewing with the unselfconscious attention of a caged baboon, goofing with the stage lighting, flailing about with gestures at once feeble and violent, Krapp is as colourless and decrepit as his surroundings. Even his clothing is monochrome – filthy black waistcoat, scruffy collarless shirt, fusty old kecks and, incongruously, white slip-ons, relics of nattier days.

The faltering heartbeat of this living death is the man's sound archive diary scooped into boxes, metic-ulously numbered, shambolically stored. Laboriously seeking out box five, threading up spool three, he is thrown into a rage by the plummy and self-satisfied echo of his voice at age 39, 30 disillusioning years ago. Back come the women: Bianca – were the white shoes for her? – the effort that was Effie, the phantom-like Fanny. Back too, the confidence of relative youth, the showing-off, the way with words. "Viduity" he could once drop in with panache: now he has to look up its meaning and settle for the simple, enjoyable howl of the word "spool". In the midst of life we are dying, and the brain cells are dying most of all.

Morbidity is Samuel Beckett's calling card, and it comes in a variety of greys. In Happy Days, Winnie, in some ways the female counterpart to Krapp, twitters on, up to her armpits in sand, topping up her make-up, hoping for the best. The has-beens or wannabes of Waiting for Godot are energised by fear and expectation in their no-man's-land. But for Krapp, the game is up. Gambon, his face hollowed out by loss, is held hostage by his own erotic reminiscence, as shattered by what he hears as a man contemplating his assassin. The horror is, his life is over, but it is he himself who has brought it to its shabby end. When he drags out the archaic microphone for his annual recording, he can barely utter. He looks balefully into the machine, as though the younger man were actually inside, shocked by his youthful error of judgement, his belief that there was better yet to come, and powerless to stop his own decline.

The good news is, it doesn't have to be like this: Gambon, at the age of his character, is at the height of his powers, shaping every word of this spare and suggestive text with skills honed over a lifetime, painting in 50 minutes the detailed portrait of a sorry soul. And that's not a man on his way down.

'Passion' (0844 871 7632) to 27 Nov; 'Krapp's Last Tape' (0844 412 4659) to 20 Nov

Next Week:

Kate Bassett flies to Birdsong, an adaptation of Sebastian Faulks's novel