Hold your piece: If you recognise their voice, they're not doing their job. Jasper Rees on the mellifluous, sensitive, altruistic art of the documentary voiceover

In the worst nightmare of Mark Halliley, millions of viewers watch a documentary because it is narrated by Mark Halliley. The name may be unfamiliar, and so is the voice, but if you've ever watched Cutting Edge the chances are you've heard him. While all around were losing their heads in The Impossible Job, the gripping story of Graham Taylor, Halliley was keeping his in a narration characterised by neutral tones and isolated calm. The same went for Exposure, the hair-raising film about John Ridgway's survival course for executives. This was a voice you'd never spot in a crowd, 'a Home Counties middle of the road voice', says its owner. 'I'm not a voice that people particularly notice and that's why they like it.'

A sizeable majority of voiceovers for television documentaries are done by people you've heard of, but almost never are they there to draw attention to themselves. 'I would think it would be a fairly specialised viewership that said, 'I will watch that programme because John Shrapnel's voice is on it,' ' says John Shrapnel, who has done a lot of work on natural history films. That specialised viewership would probably contain his immediate family and his agent.

According to John Sparks, the series producer of BBC 2's The Natural World, 'Some producers feel that if they can get a five-star name to go with their film somehow that helps to promote it and actually it doesn't. Viewers know quite well that they're just being got in to narrate the movie.' So when you see that a very well-known actor is narrating a film about giraffes or hurdlers or drug squads, they have been hired for reasons other than fame, even when it's Dirk Bogarde narrating last Sunday's Schindler or Ian Holm narrating last night's Beyond the Clouds or Juliet Stevenson narrating anything to do with hospitals or prisoners of conscience or any other film that needs a caring, concerned voice.

Well, usually that's the case. Lorraine Heggessey, the producer of BBC1's The Underworld, hired Bob Hoskins to read the script. 'I'd be lying if I didn't say that I thought he'd attract viewers. You use whatever you think is going to make people watch your series.' But he wasn't taken on for specifically promotional purposes. The hiring of Hoskins is a perfect example of the marriage between narrator and subject: he has portrayed cockney gangsters galore, so he was the obvious choice to narrate a series about cockney gangsters.

'What I wanted was an East End type of accent,' says Heggessey, 'and I thought if I just got anybody to do it, it would sound put on, and if I just employed an anonymous actor or voiceover artist whom people hadn't heard of before then they might start thinking it was phoney, so what I needed was someone well known, well established, who had this voice I was looking for.'

Sometimes the marriage of narrator and subject has an element of farce. Not long ago the famously large, hairy and forbidding Brian Blessed voiced a Natural World film about - yes - gorillas. When Ken McGill, who made The Impossible Job and the BBC's stirring Olympic series Tales of Gold (suitably narrated by Kenneth Branagh), was after a voice for a Belgian documentary about Manchester United he hit upon David Suchet. 'They get Poirot over there and I thought, Belgian detective] It was that crass, I'm afraid to say.'

Often the choice is much more agonised. When Robert Fleming, the maker of Flying Squad, Murder Squad and the recent Scotland Yard, needed a voiceover for a documentary about the treatment of paedophiles, finding the right voice was a challenge. He plumped for Aden Gillett, who plays Jack Maddox in House of Eliott. 'We clearly couldn't be seen to be too sympathetic and yet the subject had to be handled sensitively. If you close your eyes and listen to Aden he's got a marvellously sensitive voice and the right quality of strength.'

Mostly, though, the selection procedure is simple and practical: who's got a nice voice, and is he available? Less often do they ask if she is available, because documentary voiceover work is an area where women - apart, it seems, from Juliet Stevenson - are deprived of equal rights. This is mainly because producers of all those films about violence on the plains of Africa or the streets of London tend to assume this is men's work. 'I personally find that women lack a sense of authority in their voice which I like to hear,' says John Sparks, though he concedes that Josette Simon's voiceover for last Sunday's ravishing film about flamingoes did hit just the right elegant tone.

Usually what producers are looking for is something mellifluous but not obtrusive: grit but not gravel, flavour but not fruit. Choosing the right voice is vital, because anything that hits the wrong note can easily foul up the whole show. A recent Natural World film about arctic foxes was narrated by Bob Peck. 'He's a bloody good actor,' says Sparks, 'but didn't do much for the film. All I could hear was Bob Peck. I know Bob Peck is not terribly involved with arctic foxes, and I didn't think it was as successful as having, say, Shrapnel or Martin Jarvis - a nice, story-telling voice that's easy to listen to.'

The best voiceover artists know exactly what's wanted of them, and what isn't. 'There's no way that I would ever be tempted to try and impose a very idiosyncratic instantly recognisable read on to a nature film,' says Shrapnel. 'I think I can get the story over quite effectively and quite gently and the viewer will in a sense never know it's happened.'

Philip Tibenham, the narrator on Scotland Yard and The Downing Street Years, is, like Halliley, a television producer in his own right. When producers choose his classless voice it is usually to hint that you're not listening to any old luvvie but to someone connected with the making of the film. Also, as an insider, he is instinctively in tune with the technical demands of his producer. 'You normally have X number of seconds to say what you've got to say. So the sort of thing you're told is 'do it exactly as you've just done it but a second shorter'. The trick is to sound as if you're talking at the same speed while getting either more or fewer words in.'

It is the possession of such skills that support Mark Halliley's claim that doing voiceovers is no more than a craft. 'It's more difficult than it sounds. But I'd hate to sound as if I was some sort of artist.' If he did sound like an artist, he'd be out of a job before you could say 'That's the unmistakeable voice of Mark Halliley.'

(Photograph omitted)