Ivanov was a canny choice of Michael Grandage's, the Donmar's artistic director. It is recognisable but not familiar Chekhov. The first of his plays to be staged, it's compressed in feeling, and in space: it suggests a society that's bouncing off its own walls. Written at speed, it was revised at leisure. Chekhov was taken aback by people grumbling that they couldn't see why anyone would bother with the central character: he called it his blockhead play. Actually, though flawed, it's a truly important piece of work. It sends up a plea for looking rather than judging. It is one of the best ever dramatic portrayals of depression.

Ivanov could be the love child of Hamlet and Hedda Gabler. Chekhov's hero-antihero is clouded by anxiety. He's in debt; he's fallen out of love with his wife, who left her family and her Jewish faith to marry him and who is now seriously ill: Gina McKee memorably captures the pallor and the febrile gaiety of a tubercular patient. He feels guilty, but also remorseful about not feeling sufficiently bad. He's in a miasma of melancholy, agitation and resentment. But he commits an act of violence which is muffed by most Chekhov characters.

It's one brilliance of Tom Stoppard's light-on-its feet, ingenious but not too pleased with itself translation that Ivanov's condition - the thing that has turned him from being an idealist to a no-hoping no-hoper - is everywhere described and nowhere diagnosed. It's a sack on the back, it's a sulk, it's a melancholy which women want to cure. The mystery becomes part of its torment; it is constantly escaping, changing shape, never treatable.

You could call it a general loss of point, a seeping away of savour. The play is in part a clinical study: Chekhov wrote it when he was writing short stories such as 'Typhus' about illnesses that engulf people as if they were moods.

Ivanov's infection spreads its germs everywhere: everyone in his world is fighting off lassitude. When the curtain goes up on the second act it shows a sitting room paralysed by gloom. The first movement is a yawn. The most repeated word, murmured from a card table, is 'pass': its lethargic syllable punctuates the action with the precision - this is a genius piece of direction - of a string quartet. In the course of the evening, which is inflected by Adam Cork's Klezmer-influenced clarinet and string score, you see the hope draining from Christopher Oram's exemplary design, which begins with the artificial brightness of a turquoise sky and shrinks to a mud-coloured sitting room, dully lit (the hostess is too mean to pay for unnecessary candles) and mired in gloom.

It has become commonplace to talk of Chekhov's plays as being secretly funny, and too lugubriously paced in British productions. Yet Ivanov truly is comic. It has romp and high farce: its movement, a switchback between pathos, fear and hilarity, is quicker, cruder and more violent than the most celebrated Chekhov plays: Chekhov said he wanted to give the audience a sock in the jaw at the end of each act, and he did. There is a crying quartet which turns into high comedy. A confessional soliloquy by the hero is continually interrupted by his father-in-law dipping in and out. You don't know where the play is going to swerve in tone: it lurches unpredictably. The hysteria of the movement is the point.

This is a play that mimics the moods of its hero: it flashes and changes, goes up and down, in and out of sense, as if it were being pushed out of control. Its sympathies are unexpected. A doctor, who tries to protect Ivanov's wife, and who looks, in the rising star Tom Hiddleston's interpretation, very much like a young Trotsky - wispy, bespectacled, earnest - is accurate in his denunciations, but discredits himself by making his honesty into a badge of honour. Its glories are various. Malcolm Sinclair as an ambiguous uncle is one of them. There's a very funny drunken discussion about the best things to eat when you have the munchies; there's a terrific Chekhoppardian description of the weirdness of the central character. In this version he makes the world look not, as Constance Garnett has it in the old 20th-century version, like a 'museum of curiosities', but like 'a sort of modern art gallery - I look at things and don't know what to make of them.' It might be possible here to think of Nicholas Soames and a Damien Hirst stuffed animal face to face.

Kenneth Branagh, artistic associate in the project, returns to the stage as Ivanov. And triumphs. He begins as an unlikely depressive: so bluff in face and body. Which makes him the more convincing as someone gripped by illness: he seems to be descended on, taken over by something which is to do with himself but not explained by himself. He is by turns stunned and disgruntled; when he crumples, falling on his feet, at an act of generosity, it is as if he is being unravelled.

Vox pop

Is Tom Stoppard haing a laugh with Chekov's tragedy

John Plummer 60, tourist

It was incredibly well done, all the characters were very well cast. Tom Stoppard's adaptation made the play; the original script itself is getting a little dated. I found the added satire very refreshing.

Eamon Ali 20, student

It was fantastic - very sparse but very powerful. And really well staged to the point of having a filmic effect. It's great that you got the sense of Slavic society without everyone having to resort to comedy Russian accents.

Beth Goldstein 53, professor of education policy

The script itself was a little surprising; it's not the Chekhov that I'm used to seeing. It had a farcical quality. It's interesting how Stoppard works with the anti-Semitism of the period in the narrative.

Otty Stride 18, unemployed

I found the open references to Hamlet one of the most striking elements of the play. The fact that the author had the courage to walk in Shakespeare's shadow was incredibly powerful. I preferred Mamma Mia! though.

Kiran Kheta 32, lawyer

The whole cast was brilliant. The dramatic structure didn't have its foundations purely in any one of the characters, although I did especially enjoy Kenneth Branagh's Eeyore-like Ivanov.

Interviews by Charlie Gilmour