In a cynical, marketing-crazed world, it is probably absurd to be distracted by three words on a promotional poster.

All the same, one or two people might have registered the mildest start of surprise at seeing a quote which appears beside the head of Stephen Fry in advertisements for his new one-man show. It reads: "'A towering genius.' Melbourne Age."

If this is one of those clever, semi-ironic, self-mocking jokes, it has backfired. Fry is now so loved by his fans, representing wit and sanity in a mad, humourless world, that the attachment of the word "genius" to what he does poses no problem whatsoever to them. Presumably, though, as someone who cares about the use and misuse of language, the man himself might have wondered mildly where the genius resides. He writes funnily and well, is an engaging performer, and he has the nerve to speak up bravely about matters of public interest – the Pope's visit, the hypocrisy of the press, timorousness at the BBC, and so on.

None of that quite amounts to towering genius. Here, the original Australian review from which the quote is lifted (and quietly given an indefinite article, cranking up the compliment a couple of notches) is of some help. "It seems impossible that such towering genius can be more than a crafted pasteboard mask," wrote the Melbourne Age's John Bailey, "but if the Fry we follow is something of a lie, it is of the sort his personal messiah Oscar Wilde would approve: a lie that speaks a greater truth. In this case, Fry proves we can take power over, and joy in, the role that is ourselves." Now we are getting somewhere. Fry's genius lies not in any conventional work, but in being Stephen Fry. It is a strange and very contemporary kind of expertise: the highly successful creation of a public self.

Schoolchildren have learned this basic fact of modern life. They want simply to be famous. Ask them what they want to be famous for, and they will be genuinely confused. What is Jordan famous for? Or Paris Hilton? They are simply themselves. Creating a public persona is trickier than it may appear. It requires the right balance between reality and marketing, between the authentic and the created. An element of vulnerability needs to be in the mix, crossed with a capacity to take the knocks. You need to be able to show just enough of your own personality – a loss of temper here, a touch of mild vanity there – to remind fans of your humanity.

On the other side of the compact, the fan gains virtual friendship (more valued sometimes than the real thing) and a sort of reflection of themselves – only wiser, wittier and more successful. To read some of Stephen Fry's fan mail on his own website is to realise how much he means to his followers. What seems to make him genuinely loved is that he neither looks nor sounds as if he comes from another planet, occupied exclusively by the brilliant and the beautiful. He is Everyman writ large, ordinariness made extraordinary.

Few manage this creation of a parallel public version of their private selves with any degree of success. David Beckham did it brilliantly. Tony Blair, in the years before 2003, was a master of it. A few authors – Martin Amis, Zadie Smith, Bret Easton Ellis, Jeanette Winterson – pull it off while others, notably Kazuo Ishiguro, work hard to avoid doing so.

Quite often the public persona becomes hopelessly confused with the real thing. At one point, Ricky Gervais appeared in interviews to have become the sneering bigot he parodied in his one-man shows. When he tried to explain that that he was impersonating a role, an arrogant celebrity-Ricky who has let fame go to his head, it was too late. Everyone was too confused to care.

The recently released film I'm Still Here is a disturbing portrait of what happens when a constructed identity falls apart more dramatically – or, rather, ceases to be the identity which the public has chosen for it. Purporting to be a fly-on-the-wall documentary, the film follows, often uncomfortably, the psychological meltdown of the actor Joaquin Phoenix after he has decided to abandon his acting career in order to become a (hilariously bad) rap singer. Although a few of the scenes in the film have a whiff of fakery to them, the general direction of the various rows and acts of humiliation and cruelty seems entirely convincing. It is, we are now told, a spoof. According to Phoenix, he and his director Casey Affleck "wanted to do a film that explored celebrity and explored the relationship between the media and the consumers and the celebrities themselves". They succeeded, and, weirdly, the fact that the whole thing turns out to be a clever hoax does little to undermine its effect. There is a tug of voyeurism and cruelty, it suggests, in the relationship between an object of fame and his public. Once the star steps out of character and the genuine human being reveals himself to be all too genuine and human – that is, a pain in the arse – admiration quickly turns to contempt.

The film shows the nastiness of online critics and the show-business establishment in the face of what appeared to be mental illness. Indeed, by playing a heartless game with the audience, Phoenix and Affleck are part of the same process. In the end, everyone feels less. Any sympathy for the suffering actor turns out to have been misplaced. He was having a laugh at our expense all along.

Clearly, the creation of a public version of the self is more than mere marketing; it is a method of survival. The effect on the rest of us is more debatable. By putting inverted commas around everything which is said and done, the process turns human beings into consumer items, with all the contradictory, embarrassing stuff of normal, everyday life removed – or at least buffed up and glamorised.

When the image slips, as it does (or appears to do) in I'm Still Here, the reaction of the world outside is ugly. We want our celebrities to be vulnerable, but not that vulnerable. When they do the unforgivable, and show contempt for the PR-fuelled merry-go-round of celebrity, there is a bitter sense of betrayal.

It is, in other words, a game, not a lie that tells any greater truth. The idea that a famous person can, by acting the part, show ordinary mortals how to gain control over their lives is a comforting and slightly dangerous illusion.