Cue the usual sneers. Ever since the first camera bulb flashed, there have been complaints from the rich, beautiful and envied about the heart-breaking loneliness of fame. If a film star truly likes the idea of being a tiny, insignificant speck, it could be fairly argued that the place to find out what it is like is not in the foothills of the Himalayas but on the train with smelly commuters during the rush hour. Just as the public may be seduced by a fantasy version of fame, so the famous are prone now and then to romanticising the joys of ordinariness.

But there is more to this cri de coeur than actorly showboating. Anyone doubting Knightley's claim that "the celebrity thing is completely crazy" (her thinness has recently been associated with the debate surrounding anorexia) need look no further than the recent life and cuttings of Kate Moss.

It is pointless to blame Kate Moss for a bizarre situation in which the worse shape a person appears to be in, the more successful, interesting and desirable she will become. The sickness seems to lie within a ghoulish public, hooked on sensation and misery in the lives of the famous. Now bored of drama and stories that have been made up, people are hungry for real pain and real joy as experienced by real, preferably attractive, celebrities.

Here is the joy of the downward-spiral story: once it is seriously under way, an orgasm of public emotion is virtually assured. A story like that of Kate Moss and Pete Doherty can lead to tragedy, opening the floodgates of Diana-esque guilt and recrimination, or to redemption, with joyful confidences shared on a chat-show sofa, a heart-rending autobiography and general gladness at the fortitude of the human spirit.

Whether the public tears are of sympathy or of joy matters little, so long as that sharp moment of shared feeling has been delivered.

Keira Knightley has discovered that the problems begin when the desired public figure is not obligingly self-destructive. Such is the lust for second-hand pain that the public has become like a punter in a house of correction, with the media acting as an eager Madam Whiplash. If the tragic celebrity of the moment turns out not to be tragic at all, the press will do its best to remedy the situation.

So an actress who, until that moment, has been annoyingly balanced and sane is reported, probably on a website for weirdos, to be anorexic. Even if the actress wins a libel case and proves in court that the story-line chosen for her is a lie, the game has begun.

She may not be anorexic herself but now she is associated with anorexia in the public mind; when a teenager starves herself to death trying to look thin, the name of the actress is invoked. She has become the cause of misery, even death, among those that admire her. How does that make her feel? she is asked. If she still clings doggedly to the illusion that she is not tragic but normal, then the next step will be to photograph and report her every move . . . eating, not eating, trying (very suspicious, this) to hide from cameras. Eventually she will crack, or play along with the game.

It is everywhere, this hankering for a drama which will provide that sharp, bracing hit of public emotion and confirm some plonking verity of the moment about addiction, ambition, desire and, of course, fame. People like Knightley might with justification blame the media but, as every Madam Whiplash knows, the true malaise lies within the punter.