Matt Trueman: Cleansed is more than just shock theatre

Twenty-one years ago, the Daily Mail's theatre critic Jack Tinker left the Royal Court Upstairs and called his news desk. There was, he explained, a story to be had in a show that had just opened in the studio theatre – a play so explicitly violent that it was worth reporting on as well as reviewing. Just like that, Sarah Kane's debut play, Blasted, became "the theatrical controversy of the decade."

For the next fortnight or so, there were news stories, outraged opinion pieces and staunch defences, even cartoons and a Newsnight debate. You know the headlines: "this disgusting feast of filth" and so on. The play's very existence became a question of public morality. Tabloid journalists set out to track the playwright down. The Royal Court employed extra staff to manage the press.

Twenty-one years later, we're off again: same playwright, different play. Katie Mitchell's National Theatre production of Cleansed has triggered similar news stories after a handful of audience members fainted during previews.

Putting my journalist's hat on, it makes sense. It's not every play that has audience members fainting. When it happens, even if only five out of 2,000-odd people pass out, you've got yourself a news story. It's an extraordinary occurrence. Add a substantial number of walk-outs – 40, in this case – and the case for publication becomes stronger. Incidentally, by press night, only nine people had walked out of Blasted – albeit in a much smaller theatre.

That doesn't mean it's not dismaying, though, and if I put my critic's hat back on, I'd argue that framing Cleansed through its violence alone is entirely reductive. It turns the play into a gauntlet to be run by audiences – a kind of London Dungeon endurance test. Can you handle the horrors? Can you remain conscious?

Some of the reviews follow suit – fascinatingly so, in fact. In the Times , Ann Treneman catalogues the violence: "incest, eye-injecting, tongue-cutting" – etc etc – "and, oh yes, book burning." The last is written with a roll of the eyes, but the act of listing is itself a way of neutralising the play's contents. To list acts of violence so abstractly and detachedly is to decontextualise and depersonalise them. Compare Treneman's "throat-cutting" to Kane's stage direction: "Tinker cuts Rod's throat."

In the Daily Mail, Quentin Letts goes further. He provides a running order, complete with timings: "At 67 minutes, a dancer in a shower is forced to part her thighs. At 70 minutes, a young man in a red dress is force fed a box of chocolates." Consider the implications of that: Letts spent the play checking his watch. He does so as early as 22 minutes in. That's fascinating. How better to disarm the play than by refusing to give in to it? Letts keeps himself at a safe distance from its contents. By checking his watch, he keeps in touch with real time and the real world rather than submitting himself to those of the play.

That is, in effect, an enactment of Treneman's final point – one that actually unravels the whole play rather brilliantly: "It's shock theatre, in yer face, as they call it. It doesn't mean you have to go."

However, to call Cleansed shocking is too simple. It's hard, maybe even impossible, to be shocked if you know what's coming. Not only is Cleansed a revival – a contemporary classic, no less – Katie Mitchell does something rather smart with the violence. She makes us brace ourselves. She conditions us, as an audience, to expect it. The violence falls into a pattern: Tinker – the torturer – rings a buzzer, orderlies arrive bearing implements, wheeling on a hospital bed or a metal chair, laying out plastic sheeting —and we know what's coming next. In time, the anticipation becomes more potent than the violence itself.

Besides, that violence isn't straightforward. Mitchell toys, beautifully, with theatre's paradox. The more convincing the violence, the less convincing it seems. The better the stage trickery, the more evident the trickery becomes. Show me mangled limbs, and I'll see make-up. Show me sex and I'll see simulation. Mitchell's keenly alert to this. She and designer Alex Eales punctuate the play with reality: flames, nudity, cross-dressing, even visible microphones.

For me, the hardest thing to watch is that force feeding: a real actor really eating real chocolates – night after night after night. You hear his throat clagging, his gasps for breath, his jaw chewing and tiring. You wonder if he'll puke. You worry for his health. You question his consent. He – Matthew Tennyson – seems hostage to the play, tortured by his fellow actor, by the director, by the playwright.

If one does go, it's worth giving in. To watch Cleansed is to sit alongside it. You have to let it go to work on you, on your emotions, on your subconscious, on your sense of self. It's not about something per se. It is something. You don't understand it. You experience it. You feel it. You live it.

Personally, I found it deeply troubling – and not simply because of its violence. A week on and I'm still turning it over, trying to make sense of it, trying to reconcile its contradictions, to disentangle its beauty from its violence, to process feelings of disgust and delight. It is as beautiful as it is awful; just beautiful enough, perhaps, to keep you watching.

Kane's play is usually discussed in terms of love: Tinker is putting love to the test, pushing it to breaking point. He tortures Carl to see if his partner Rod will renounce him, for instance. He operates on Grace, grieving for her brother, and turns Robin's love for Grace back on itself. Kane's inspiration was a Roland Barthes quotation: "Being in love was like being in Auschwitz."

However, by keeping Grace onstage throughout, Mitchell shifts its balance. To Kane's recurring stage direction, 'Tinker is watching,' she adds another: 'Grace is watching too.' It brings gender to the fore, and makes Tinker all the more male, all the more straight, and all the more ashamed. You see the 'otherness' of all his victims, their queerness, their ambiguity. Grace ends up both male and female. Robin is a boy in a dress. Rod, mute and mangled, is forced into a dress. All this in a university, a space that shapes (or conditions) the young.

Watching as a straight, white, university-educated man, it felt like a direct accusation – and that's both intensely uncomfortable, and vital.

Cleansed runs at the National Theatre until 5 May.