The TV crew was desperate to talk to this man. He was not keen. So how come he eventually said yes?

Thinking Aloud Cambridge Kettle's Yard

In October 1991 President Mikhail Gorbachev flew to Moscow from the Middle East peace conference in Madrid. In front of him was a written request from the production company, Brian Lapping Associates, to participate in a BBC historical documentary called The Second Russian Revolution. The programme had pursued the Soviet leader without success for two years. Around him sat his interpreter, his press spokesman and his foreign affairs adviser. The programme makers had interviewed each one of them. As they all recommended he take part, it must have become apparent to the Russian leader that he was surrounded. He duly granted the first of his many documentary interviews.

Earlier this year, a film crew turned up at Harefield hospital to get a few minutes with Professor Magda Yacoub. It was their tenth visit. On the previous nine, Yacoub had been busy. This time, towards the end of a difficult nine-hour, open-heart operation, Yacoub downed tools, emerged from theatre, and gave a short, methodical account of his brief, medical relationship with Eric Morecambe. He then went back to finish the operation.

These two vignettes would appear to be worlds apart, but they have one thing in common. They both illustrate techniques that documentary filmmakers use to sign up their interviewees: Gorbachev was snared by a campaign of encirclement with almost military precision; Yacoub succumbed to relentless patience and persistence. As a viewer you tend to take these techniques for granted. If General Giap turns up on your screen to talk about Vietnam on Cold War, as he did earlier this month, you don't give a thought to how he got there. Nor why on Omnibus Eric Morecambe's family choose to speak about the private individual for the first time on television. Many people who agree to appear in anecdotal, eye-witness documentaries do so without much encouragement: it may be a good fillip to the ego, or a chance to salve their conscience. But there are others who need to be persuaded that it is in their interest.

For these reluctant interviewees, there exists a set of complex and highly adaptable courtship rituals which apply equally to arts strands and historical series. Thus Paul McCartney contributed to Arena's forthcoming study of Brian Epstein for much the same web of reasons that George Blake granted an interview to Cold War. They both had things to get off their chests and into the communal archive of television, and were persuaded that there would never be a better opportunity to do so.

Stressing the archival nature of their work is usually a programme maker's opening gambit. The documentary has usurped the book as the place where the world assesses its own past, puts its own side of the story, confronts its own demons. Bill Eagles, who made the Morecambe film, says he was "looking to preserve the value of the estate and even enhance the value of the material". The carrot dangled in front of Cold War's participants was the chance to talk, more-or-less unmediated, to students of the period 100 years hence. "If they wanted to go on record," says series producer Martin Smith, "and leave something which would not be tampered with, all of the interview would be available for posterity in the archives unedited."

But while the impeccable credentials of the genre are one thing, trust for individual filmmakers is quite another. Clearly it helped that McCartney had appeared in three previous Arena films going back to 1982. "I'd like to think that one of the reasons he made a film was that we had a relationship with him," says Anthony Wall, the editor of Arena. Likewise Fine Art Productions, which made The Downing Street Years, was able to invite Margaret Thatcher back. But not everyone can give interviews to people who have already interviewed them. There has to be a first time, in which the interviewee can merely hope that he is not entrusting his store of recollections to a muck-raking revisionist. "We want to be invited back," says Jonathan Lewis, who has made historical documentaries for Reputations, People's Century and Cold War. "It is no use to us to get the interview and then nobody will ever talk to us again." Thus, while a filmmaker can protest good faith until blue in the face, it helps to offer some proof.

It is no surprise that putting BBC on the letterhead is not the guarantee of quality it was. "With the diminishing understanding of what Britain is," says Wall "there is a diminishing appreciation of what the BBC is." The fact that Cold War was made for CNN would have meant more to many of its contributors. But one of Cold War's executive producers, Jeremy Isaacs, was able to point to his own CV. "If I can be immodest," he says, "of course it helped us that I had made The World At War." Anyone working for Brian Lapping Associates is able to boast that these were the people who brought you The Second Russian Revolution, The Fifty Years War and The Death of Yugoslavia. Likewise Eagles could show the Morecambes the affectionate series he made about comic double acts.

"Naturally the family would be wary that somebody might want to make something that wouldn't necessarily be in their best interest," he says. "That's not what I do."

Documentary makers talk of casting their films. As in the movies, people are often cast according to the domino principle. One person comes aboard: others follow. None of Morecambe's friends would talk to Omnibus "until they were confident that this was a bona fide operation and that the family was involved." For one KGB spy interviewed in Cold War, "it was very important to him to know that there was another very senior KGB guy who'd be prepared to be interviewed by us," says Jonathan Lewis, "and that consequently when he goes to the old spies club nobody is going to start throwing snowballs at him."

In the field of politics, though, it is as often the case that people chose to talk less because their friends have done so than their foes. That was the strength of The Fifty Years War and The Provos. It was a key factor in the elaborate sting by which the makers of The Death of Yugoslavia secured an interview with Slobodan Milosevic [see right].

One trump card these documentaries can play is that, even though they have to get into bed with some unpleasant people - unreconstructed Nazis, keepers of the Stalinist flame - it is not their brief to pass judgement. For Cold War's film about spies, Jonathan Lewis persuaded both George Blake and Aldrich Ames that he would not "wag a finger". "I had absolutely no judgements to make about whether they were good, bad or indifferent," he says. "I wanted them to tell me what they did and why they did it." Even so, it took two visits to the penitentiary in Pennsylvania where Ames is serving a life sentence to get him to talk. "He didn't want to talk to us, even though he'd agreed and signed the form, because he has few opportunities to exert power, so he did exert it. We had to go away and re-approach him."

That's the other trump card - the elasticity of the schedule: "in your own time" are the most rarely used words in television, and can be powerfully hypnotic. The Morecambe family took nearly two years to agree to do Omnibus. "There was no hurry," says Bill Eagles. "They had no urgent need themselves for the film to be made so they were able to discuss and review the general pros and cons." Fidel Castro kept Cold War hanging on for the same period, and only towards the end did it become "a question of how long do we carry on with this guy," says Martin Smith. "We kept on turning up in Cuba hoping to film him and were never told definitely yes or no. In the end you have to work out just how many more thousands are you going to spend on this man?"

Finally, of course, programme makers sometimes hit a brick wall and have to take no for an answer. Sometimes health is an issue. Ernie Wise eventually decided that he was too ill to take part in the Morecambe film. But often it's just plain obduracy. No one from MI6 would appear in Cold War. "We were told that they simply didn't care if there was a misrepresentation," says Lewis. Nor would Helmut Kohl, who almost never does media interviews, or Mrs Thatcher. "She was not persuaded that it would be good for her to put her own piece on record," says Martin Smith.

Sometimes a target is beyond the tentacles of even the most thorough documentary. The Arena film on Brian Epstein includes interviews with people from all compartments of his life. "But I didn't in the end find anyone to talk about gambling with him," says Anthony Wall. "He used to gamble with Lord Lucan."

'Bring Me Sunshine: The Heart and Soul of Eric Morecambe' is on BBC2 on Wednesday 23 December. 'The Brian Epstein Story' is on BBC2 on Christmas Day and Boxing Day. 'Cold War' resumes on BBC2 in February. 'Enemies of the People', Angus Macqueen's documentary about the Stalinist camps, will he shown on BBC2 this spring.