Fly-on-the-wall victims always get bitten. So why do they do it?

Naked bodies on television being old hat, this is the winter of naked souls. Last month Janet Street-Porter had the torments of her professional life laid bare in Nightmare at Canary Wharf, a searing documentary about her disastrous year as head of the Mirror Group's Live TV cable station. This week, the Modern Times film about socialite-cum-PR Liz Brewer and her clients was repeated, with updates that revealed how enraged two of those clients, Ivana Trump and Mona Bauwens, were at the way they were portrayed.

On 16 January, Jeremy Isaacs will undergo the same embarrassment as BBC2 begins screening The House, a six-part series tracing a typical year - in other words a year of bile and fury - in the life of the Royal Opera House, where he is general director.

In all cases the attack weapon is the fly-on-the-wall documentary, and the damage is to a large extent self-imposed. Ms Street-Porter and Mr Isaacs invited the flies into their parlours and - unlike in the familiar verse - were themselves devoured. Yet it is hard to have sympathy for them, for both have spent most of their working lives as TV executives, and better than anyone should have known what to expect.

Fly-on-the-wall documentaries have been around for more than 20 years. In the mid-Seventies the Wilkins family of Reading allowed a BBC crew to film the ups and downs of their domestic life for a riveting 12-part series. All families have quarrels, but the presence of the cameras made the Wilkins' divisions more traumatic and harder to heal.

Since then dozens of institutions - police forces, schools, colleges, military units, golf clubs, prisons, London Zoo, even entire villages - have allowed cameras in to see how they operate. More often than not, it has ended in tears.

In making these programmes, producers customarily shoot 20 times as much footage as they are going to use. They select which incidents to show on the criterion that argument and disaster make better television than harmony and success.

Edward Mirzoeff, executive producer of The House, says: "No meeting is interesting unless there's a certain amount of conflict in it. When you see tempers lost, difficulties raised and people having disagreements, what you're actually seeing is the difficulty of the job they're doing and the way they're overcoming it.

"Obviously, in some senses people would like a corporate video. They believe when they go into it that they will come out pure and shiny, everyone smiling and so on. When they first see moments of difficulty and conflict, they're slightly taken aback."

So why do people allow it? Mr Isaacs gave the BBC crew a fairly unrestricted run of the Royal Opera House during the 1993/94 season for several reasons. The house is constantly under fire for being elitist and remote, a drain on public funds. Believing that it was emerging from a rough patch artistically, Mr Isaacs hoped the series would give a sympathetic picture of the high quality of its work and make taxpayers less resistant to subsidising it.

He is not convinced yet that he was totally wrong. Certainly some fine artistic moments are recorded in the six films. The overall impression, though, is of an institution sometimes out of control and constantly at war with itself and with outside forces.

A conductor quarrels with a director and stalks off a few days before opening night, meaning that a replacement has to be flown in from Canada. The Royal Opera and the Royal Ballet hire the same designer for simultaneous productions, and she runs into a time bind. A new director of public affairs is hired and has the box office manager sacked. The two barmen in the Crush Bar disclose that they have not spoken to each other for years.

There is a showdown with the stage hands' union and a strike by the chorus is narrowly averted. Mr Isaacs looks constantly harried and unsmiling. The Opera House board and the Arts Council - two classic quangos - balk at giving him the money he needs to run the place, and in his frustration he lets slip a four-letter word: Ms Street-Porter did the same when facing mounting setbacks in Nightmare at Canary Wharf.

Mr Isaacs saw the films in time to ask for adjustments. He succeeded in getting the commentary changed in a few places, but nothing was excised. "I'm not accusing the BBC of bad faith or even of conscious bias," he says ruefully, "except a bias in favour of incident."

Swearing is a recurring problem in these warts-and-all exercises - as Graham Taylor, former England football manager, discovered when he was the subject of a recent one. The veteran producer Roger Graef, a pioneer of the form, recalls making Operation Carter, a 1983 series about a Regional Crime Squad: "They swore non-stop. We negotiated. I remember the then controller of BBC1 ringing me up and asking whether we actually needed so many 'fucks'. We had already reduced 150 of them to about seven and I had to take him through every single one."

Michael Waldman, director of The House, denies that his series is unreasonably negative: "It does show that they do good work with limited resources and with passion. Obviously, the passion results in disputes, discussions, rows even, but that's the nature of any institution, especially an artistic one. People are often initially disappointed with films about themselves."

Fly-on-the-wall is in one sense a misleading term, because the cameras are always obtrusive enough to be noticed. In front of them, people sometimes act in the way they think they ought to, rather than as they do in real life. Thus when board members Sir Angus Stirling and Sir James Spooner attacked Mr Isaacs for not being strict enough with demanding artists, they were, say insiders, acting out of character. Normally their criticism would be more restrained: in public they had to be seen to be tough.

Mr Isaacs tried to exploit the presence of the camera crew in one of the lighter scenes when, getting ready for a royal gala, he was having trouble tying his bow tie. He asked for help but Mr Waldman, anxious to maintain a low profile, refused.

It was a sybmolic moment. Once you admit the intruding cameras, abandon hope, for everything is on their terms. They will decide what the public gets to know and think about you - and you won't even get your tie fixed.