Standing on the Trafalgar Square fourth plinth: Is it art?

Soon enough, my mind was racing. I could make a film. I could persuade someone to tailor an un-militaristic uniform. Maybe Vivienne Westwood would run something up? Or Stella McCartney? No, too ambitious.

Never mind; the idea was clear enough. The statues of kings and generals celebrate death and subjugation. I would fly the flag for life.

"So," says Gormley, when I meet him at his studio behind King's Cross, "what are you going to do?" There was something of the head teacher about the man, but he was dressed in white with dusty work boots.

He reminded me of Morgan Freeman in Bruce Almighty, which is to say that he looked like God. The thought occurred that, while the plinthers have the impression of freedom, Gormley is the Great Architect. He is the artist, and this is his show.

But that isn't how he sees it. "For me, the most important thing is you, as a participant. What happens to you up there is what matters most."

This sounds a little ominous, bearing in mind that the last man to make a public spectacle of himself in this way in London - David Blaine - became an object of ridicule.

"It's not exactly hanging, drawing and quartering," says Gormley, "but it's got a bit of the stocks about it. There's a public square with a body that isn't allowed to leave. This is a serious commitment.

"Your life in this hour is being bound into a social contract which links you with another 2,399 people, and you have to fulfil that commitment. It's a real bond. And, barring a heart attack or some very serious epileptic fit, you're going to have to stay up there."

This wasn't reassuring, but Gormley did offer some comfort.

He told me my slot was at 2am, a time when even a pigeon would think twice about visiting Trafalgar Square.

Gormley made it sound romantic even: I would be up there, illuminated by halogen and starlight, with only the hardiest of nutters around to disturb my reverie.

"But I can imagine that during the weekend afternoons, for example, people will regard the person on the plinth as fair game, for a bit of repartee at least. And how that works will be part of how you respond to this extraordinary situation," he says.

"I'm using this plinth, this funny old bit of Victorian cast-off street furniture as a test site for asking two questions. One is: what is a human being? The other is: what is a sculpture?"

We talk in the drawing room of Gormley's vast art factory. His assistants play football in the courtyard below and I can hear the sound of welding.

The workshop is full of Gormley sculptures: statuesque versions of himself and "energy" figures in which the human form is represented by networks of metal.

In his newer works, Gormley seems to be looking inside the body at cell-like structures and molecules. But the artist's trademark remains the amply proportioned cast of his body which he positioned around the South Bank in 2007, in a display called Event Horizon.

"Event Horizon encouraged people to look again at their city, and maybe think about it in a way that a Martian would. There's an element of me that is still poking around these issues. It's as if I'm not a member of the human race, and somebody gave me this spaceship, which is a human body, but I'm not quite sure where I belong.

"I've never quite talked about it like this, but I'm sort of asking: what is the human being? And where do they belong? Maybe they don't belong anywhere."

Gormley's popularity hasn't always been matched with critical approval. Even before One & Other has begun, some have dismissed it as an outpost of reality television, Big Brother and all that.

It is a chastening thought that by volunteering to stand on the plinth I might be aligning myself with the likes of Jade Goody and Susan Boyle, but on this point, at least, Gormley is reassuring.

"There's no competition. You're not voting people on or off the plinth. No element of what you do is being judged. The idea of this piece is to honour and acknowledge the diversity, integrity and liberty of the individual citizen.

"I will be very upset if people think they are being exploited. There is an element of willing exploitation in the members of a Big Brother house or Britain's Got Talent.

"This doesn't use the same mechanics. The whole point is to undermine and question and contradict the hierarchical established order.

"This is not about grace and favour, it's not about privilege and patronage. It's important that people realise they're not being selected for what they might or might not do."

I tell him what I might or might not do, and he suddenly becomes engaged. "It's interesting that you immediately go from being a sole individual, responsible for their own actions, to suddenly being a symbol or a focus for other people."

He is careful not to offer approval or disapproval. That would be tampering with the experiment. So I ask him straight out: how does he feel about having an un-militaristic statue on the plinth?

He answers by talking about The Medals of Dishonour exhibition at the British Museum, which aims to subvert the pomp of regal coinage.

And statues, he says, "are often about the memory of the fallen. This is much more about the future. I do genuinely hope that we're going to learn something from this, that the collective warp and weft of this will tell us something about what people find funny, what they find fearful, what they love. It will be a taking of the temperature of some collective body."

The idea, he says, is "celebrating how each of us is creating or contributing to a possible future, which is shared. That's really at the core of it, and it's saying you don't have to be dead, you don't have to be a king, you don't have to be a general, you don't have to have killed anybody; you, in your individual uniqueness, are a critical point in a shared world. And valuable."

That sounded as close to an endorsement as I was likely to get, and made the idea of standing seven metres above Trafalgar Square at 2am on a midsummer morning seem sensible and almost necessary.

Gormley had talked a lot about the responsibilities and duties of being a plinther, and of the need to prepare.

I would take a jumper, an umbrella and relieve myself before being promoted to glory by the fork-lift.

But did he have any advice? "Advice?" He chortled. "Just be yourself for everyone. Follow your own instincts."

And that's the odd thing about Gormley's plinth. My natural instincts are to stay home, do nothing and be myself for me, but that's exactly what I won't be doing at 2am on 30 July.

Alastair McKay will spend his hour on the plinth making a stand against gun violence. For more information on the International Action Network on Small Arms, see www.iansa.org

Antony Gormley's One & Other Fourth Plinth project is commissioned by the Mayor of London and produced in partnership with Sky Arts.

Sky Arts will go live from the plinth on 6 July at 8.30am, broadcasting a 24-hour feed on www.oneandother.co.uk.