At a little before 9am, today a protester scaled the fourth plinth in London's Trafalgar Square to demonstrate against actors smoking. He was followed by the first official occupant, who stood with a giant lollipop emblazoned with the logo of the NSPCC. Strangely, all this was somehow less compelling than the man in shorts and red T-shirt who came next. He had no apparent agenda at all, except being there. Most of the time, he stood near the lip of the plinth with his hands in his pockets, like a character in search of an author. His presence was what counted. Just as some sculptures have more presence than others (a tiny bronze Giacometti can somehow fill a whole room), so it is with the living.  Not everyone here will be a living sculpture. Some who are lifted on to the plinth will be living advertisements for themselves, craving attention, fame or notoriety. I expect numerous hapless performances, a bit of nudity, protests and declarations at all hours of the day and night. There's always the chance someone might immolate themselves, or defecate, urinate, masturbate or vomit. Are they allowed shoelaces or belts up there? Are they frisked for weapons or secret intentions? Is there a contingency for those who might wish to give birth, or any potential suicides? Taking a running jump, it would be easy to hurl oneself over the safety net to the paving slabs below. Anyone attempting to recreate the artist Yves Klein's famous 1960 Leap into the Void, a photograph of him suspended in mid-air above the street, should be warned – his image was doctored. And what about snipers on nearby rooftops, kids with catapults, miscreants with rotten eggs, bricks, guns? A stoning is entirely possible.  Living sculpture has a long and intriguing history. On 1 January 1901 the bullfighter Don Tancredo López covered himself in whitewash and stood on a box in the middle of the bullring in Madrid; the bull circled him but did not attack. López was a statue of himself risking death. When Gilbert and George covered their hands and faces in gold paint, stood on a table and performed Flanagan and Allen's song Underneath the Arches in a London gallery in 1969, they risked only the derision of the art crowd.  In 1974 Chris Burden spent 22 days on a platform in a New York gallery; and in 2002, the Montenegran artist Marina Abramovicć spent 12 days and nights on a platform, eating nothing and only drinking water. She slept and performed all her ablutions in full view of the public. An hour on a plinth isn't long, but Trafalgar Square is a different, far more public context, with live action from the plinth streamed on the web 24 hours a day.  So far the most memorable work since the fourth plinth was turned over to contemporary art has been Mark Wallinger's Ecce Homo, a life-sized cast of a young man in a loincloth, which appeared in 1999. The white resin cast looked like marble. Standing on the edge of the plinth, facing the square, it had more presence than the people who have so far been hoisted there; asking why this might be is a question both about sculpture, and about ourselves.  Yet Gormley's idea is a rich one. It combines a very old idea about images, and sculptures on plinths in public spaces, with the digital age and the spectacle of reality TV. We know that paying attention to an experiment often changes its outcome. Those who stand and watch have all sorts of expectations and fantasies. The square below is a space for the curious and the ghoulish, for voyeurs and louts; it, as well as the plinth, is a space of transit and for waiting, and for all sorts of performances and gestures. We are all actors here, under the watchful cameras of Sky Arts.  Gormley offers the possibility both for action and inaction. This is where the project's magic lies – and also its danger. It is probably his best work, even if it risks bringing out the worst in people. The artist has set up the conditions, and what follows is unknown.
In encouraging the public to act, react and interact around Trafalgar Square's fourth plinth, Gormley's One and Other is timely – and invokes a rich tradition of living art