l-r Harry Potter (Jamie Parker), Albus Potter (Sam Clemmett), Ginny Potter (Poppy Miller)
Harry Potter (Jamie Parker), Albus Potter (Sam Clemmett), Ginny Potter (Poppy Miller)
Charlie Gray

In Harry Potter's world, secrets are important. So important, sometimes, that lives depend on them. Harry's parents, Lily and James Potter, lost theirs when their one Secret Keeper, Peter Pettigrew AKA Wormtail, betrayed their hiding place to Lord Voldemort. When Dumbledore died (um, spoiler?), the secret of 12 Grimmauld Place died with him; the remaining members of the Order of the Phoenix, believing Snape a traitor, thought their secret was up. They're fragile things, secrets. It only takes one person, one indiscretion, one slip of the tongue to break them, and once broken, they might as well have never existed.

We are all Secret Keepers now. On leaving the Palace Theatre, where Harry Potter and the Cursed Child is playing, audiences are asked to #KeepTheSecrets as they go. Banners around the theatre beg us not to let slip; badges are handed out, little charms that mark us apart and bind us. As previews began, JK Rowling herself made a plea online: "I'm asking you one more time to keep the secrets and let audiences enjoy Cursed Child with all the surprises that we've built."

Our reviews often act as consumer guides – recommendations to go and see a show or not. Remaining spoiler free is part of that.

That poses a problem for critics. It's our job to analyse what a show is up to, and, to do that, we need to clue our readers in on the show itself. We have to paint a picture for readers who haven't seen the show and, indeed, for those that won't – and that includes giving a sense of the story itself.

Given theatre's nature – with a limited capacity, at a set time and in a set place –the majority of our readers won't see the show. All those people that can't get to London, all those that can't afford or obtain tickets, all those in the future seeking some record, we have to be their eyes and ears in the room. All the more so in the case of a show that's sold out until next May already. Our reviews often act as consumer guides – recommendations to go and see a show or not. Remaining spoiler free is part of that. In this case, that seems less important than ever. If people can't see the show themselves, isn't it up to us critics to clue them in. At the end of the day, that's a journalist's job, isn't it? To pass on secrets? To reveal things people don't want revealing?

Spoilers have become a cultural bête noir: a critic's no-no. Streaming services have changed the way we watch – not all at once, but in our own time, on our own terms. At the same time, arguably as a result, stories have come to stress the element of surprise. The more choice that's out there, the more stories need to grip. Compulsive viewing keeps us tuning in – or whatever the online equivalent is – and successful brands keep rolling on. We get to know characters, to love them, and, as such, we invest in their fate. Harry Potter does that as much as anything.

But. But. But. People want to know. When the original books came out, newspapers ran synopses as soon as possible, divvying chapters among staff for maximum speed. You see the same with the biggest television series today – papers run ‘episode recaps' online. The spoilers are there if you want them. As Marama Whyte has argued eloquently, it's on the individual to avoid them, to preserve their own viewing experience.

Cursed Child spoilers already exist. A single Google search will provide the plot in full, all its twists and turns laid bare. If that information's already in the public domain, the secret already broken, why shouldn't media outlets take their share of those hits? That's the business model, no? More readers means more ad money.

Some have said the whole #KeepTheSecrets campaign is itself about money: a marketing ploy to ensure the stage show runs and runs, and the script sells and sells. (Is there, you might ask, a contradiction in that: asking for hush, then publishing the script?) Journalists shouldn't be co-opted into that. Our duty is to our readers, not producers or publishers and their bank accounts. By that measure, keeping the secrets isn't our responsibility as critics.

If that information's already in the public domain, the secret already broken, why shouldn't media outlets take their share of those hits?

But. But. But. A review is not a recap. It is not a reveal-all. You don't need every single detail to unpick what a show is up to, nor to assess its quality – a critic's two fundamental jobs. With a bit of care and a bit of skill, one can tiptoe around those secrets and still say all you need.

That Harry Potter's producers invited critics in earlier than planned isn't just a mark of confidence. It's a smart move. We were originally due to review the official opening, next Saturday. Instead, we've all been into previews; our write-ups embargoed until a minute past midnight tonight. Rather than writing at full-pelt then, under pressure to file as quickly as possible, within an hour – max – of the curtain call, we've got a bit of breathing space.

Writing at speed, under such pressure, it's only natural to reach for plot – simply to describe what happens; to report it, not to reflect on it. That way spoilers lie. Under the new plan, not only do critics have time for considered writing, we can find subtle ways to skirt key plot points – They That Must Not Be Named.

Here's the thing: a big part of the joy of watching Harry Potter and the Cursed Child is the surprise of that story. Having finished the last book three hours before going in, I was genuinely excited to find out what happens next. What became of Neville Longbottom? Who was running Hogwarts? Whether Voldemort might return and, if so, how? When key plot points dropped, there were huge gasps through the audience – gasps like I can't remember. That's a rare thing in theatre. As is a plot so captivating that it holds an audience rapt for five full hours, keeping you guessing all the way through.

As I've said before, Harry Potter and the Cursed Child could do for theatre what the original books did for reading. Already, over half of the audiences are first-time theatregoers, according to producer Sonia Friedman. If Harry Potter's good – and you'll have to wait for my review to find out – how many will come back for more. That's an extraordinary thing and, as a critic – no, as a theatre lover – I want those audiences to have the best possible experience: spoiler-free, surprising, magical. That's why I'm keeping schtum – not for JK Rowling's sake, not for Potterheads, but for theatre. That's why I'll keep the secrets. Silencio.

Harry Potter and the Cursed Child runs at the Palace Theatre until 2017.

Head to our Harry Potter Guide for all you need to know about the show