It was, his critics would undoubtedly observe, a classic David Frost opening. "We were once, you and I," he told Lady Thatcher, "together at a performance of Sunset Boulevard." It was a piece of archetypal Frostiana - cosy, fawning, name-dropping with its implied sense of "We're equals, you and I and Andrew Lloyd Webber."

And what a club it is he aspires to. This, after all, is the man who concluded his interview with Newt Gingrich on Sky by actually wishing him good luck. But then his enemies would say that. It is all terribly unfair, according to the former defence minister, Alan Clark, in his review of the latest volume of the Thatcher memoirs in this newspaper at the beginning of the week. Frost, he proclaimed, is the "silkiest and most intelligent of interviewers", craftily able to draw from his subjects far more than television journalists of a more rugged breed.

So which is true? Is this the subtle new future of political interviewing? Or is it "Welcome to Hello! TV"? You could be forgiven for thinking the latter in a week which also saw an emetic piece of on-screen flirtation between Selina Scott and comeback-kid Donald Trump. Not to mention a TV interview with Michael Jackson, in which the singer tried to demand that the thing take place inside an Egyptian temple in one of America's most prestigious museums.

But there can be no doubt that old Frostie produced the goods. There was the No, No, No to Europe, which she said her successor had turned into a wimpish Yes, Yes. There was the attack on her former cabinet colleagues for espousing non-Tory cuts in mortgage tax relief. There was her over- hasty refusal to say anything at all about her son Mark. There was her bald rewriting of history on how she hadn't really signed the Single European Act et al. There was the self-satisfied I-never-took-the-whip-away-from- anyone.

And many more, capped by the messianic eye-glinting: No, I would never return, unless "there was a terrible crisis of such enormous magnitude that you would have to think about it". Heaven forfend.

David Frost is quite clear about all this. "You can only judge a question by the answer it provokes," he said yesterday. "Pointlessly confrontational questions can be self-defeating. The idea is to open people up, not shut them up. You can ask the most testing of questions in a civilised way. The late John Smith once said to me, `You have a way of asking beguiling questions with potentially lethal consequences.' That'll do me as an epitaph."

John Smith and Alan Clark are not a bad spectrum of endorsement. His peers are not so convinced. "If you're accepting flattery from a politician, you're dining with the devil and you need a long spoon," observes David Dimbleby wryly. "If he is so deadly, why do they all seek so assiduously to be interviewed by him?" mused Brian Walden from his Channel Islands home where, he insists, he is now in retirement from TV interviewing.

Frost's technique, which was pioneered on radio by the one-time crooner Jimmy Young, is to appear cosy and chatty and ask the plain person's question. "They are both extremely shrewd," said the political adviser to one senior minister. "They have done their homework and can ask tricky questions but their manner is friendly and reassuring. The atmosphere is not threatening." Apologists claim this produces the goods, as when, on the JY Prog, John Major was asked whether or not a man with so little education was really properly qualified to run the country. "It was a breath-taking moment," recalls one seasoned political commentator. "It was a piece of real cheek and Major had to answer."

But others see grave dangers in the idea that it is the plain question which really tests the mettle. "It's an extremely dangerous myth," said Michael Wills, the executive producer of Goldring, the heavyweight current affairs programme in which Mary Goldring has taken the Jonathan Dimbleby slot for the summer. "It is a terrible abandonment of politics to showbiz."

The television interview has always been a chameleon creature. For years it was simply a solicitous "Have you anything you would like to say to the nation, prime minister?". Robin Day and Ludovic Kennedy changed that with their readiness to ask the awkward question and, as time went by, to keep asking it. At first the process was spontaneous, but in the 1970s John Birt and David Cox at Weekend World developed a whole new technique, almost Socratic in conception, in which they plotted a series of questions to which the only possible answer was Yes or No and from which it was possible to plan the next question in advance. Thinking many moves ahead they turned the interview process into an intellectual edifice. "For one Kinnock interview in the early Eighties, we worked out a net of 93 questions, depending on which way he turned," recalls one of the Weekend World team.

The master of the technique was the former MP Brian Walden. "He was like a chess grandmaster thinking 20 moves ahead," remembers another colleague from the programme. "You could virtually pre-script it - if we ask this and then that, we may actually get Thatcher to say that she believes in Victorian values." She did.

In his heyday, there was no one to touch Walden. He was remorseless, leaving nowhere for a politician to hide. The approach became the new orthodoxy, though Walden began to be parodied for asking questions 10 times as long as the answer. Then the politicians wised up. They were tutored and became much more expert in blocking the process.

The chief inheritors of that approach today are John Humphrys of the Today programme and Jeremy Paxman of Newsnight. They work on much smaller canvases - dealing in minutes rather than in a full hour - but they follow the same process whereby the interviewer goes with the logic of what his subject is saying but pushes it that bit further. And this is at the heart of why the style of political interviewing is significant. There are basically two kinds of political interviewing, according to David Cox. "There is the In the Psychiatrist's Chair approach, where you get people to blurt things out. Desert Island Discs uses this, lulling people into a false sense of security by making them think they are talking about records. Revelation is what you are after. Sue Lawley is very good at this.

"Then there is the Humphrys/ Paxman approach where you are trying to find out whether someone's position stands up to scrutiny and whether they can defend it. Frost tries to do a bit of both. And that is why in the end he is unsatisfactory."

It is an analysis which David Dimbleby shares. "David Frost's interview with Thatcher was very effective," he said. "He didn't get into an argument about whether what she did was right but it produced some extraordinary insights in all that pent-up aggression." Or as someone more ungallant observed: "The name of the game is getting the barmy baroness to be as barmy as possible." For that you have to tickle her prejudices rather than accuse her of being the author of all ills in 1990s Britain. "There's no point interviewing her as though she was still prime minister," Dimbleby adds. "You don't want to cross-question her about the single currency. But if you had John Major on the same point you would want to pursue policy in some depth."

The nub of the question, according to Brian Walden, is what we mean by political interviewing. "You can talk to a politician and call it political interviewing. It's the bums-on-seats school of interviewing: we've got President Clinton in the studio; now, sir, what would you like to talk about? It hands the agenda back to the politician." The nadir of this, though Walden is too polite to name names, is the politician-on-the-sofa, Anne & Nick school of politico-chat in which, as David Cox puts it, "ill- prepared half-witted interviewers allow politicians to peddle half-truths designed to deceive the public".

At crisis times, both John Major and Tony Blair have been happy to resort to a forum where they can set out their case without interruption. "It's impressionist politics - he seems like a nice chap so I think I might vote for him," said Brian Walden witheringly. Have you anything else to say, Mr Blair? The danger is that the soft approach will drive out the more rigorous. "One doesn't want to sound pompous," said John Humphrys, "but a well-informed interview conducted by a persistent questioner does contribute to the democratic process."

Humphrys, in addition to Today, conducts the BBC's weekly in-depth TV interview, On the Record. It is solid stuff. Recently he subjected Gordon Brown to an hour's grilling on Labour's supply-side policy: how committed is it really to training and investment? Why should the shadow chancellor put up with that if the sofa beckons?

"It's a real danger," said David Dimbleby. "The BBC has many outlets, therefore it is important for it to decide, as a newspaper editor would, where certain things should be placed. Some politicians should be offered Panorama or On the Record live and told it's that or nothing. If the BBC is going to keep the heavy interview going, it may from time to time have to deny access to other programmes. The BBC needs to take institutional decisions on things like that."

If it does not, David Cox predicts, then On the Record and its like will be dumped by the BBC just as ITV dropped Walden. Even David Frost is unhappy at that thought. "There should be room for all the ways of informing the viewer about those in power," he said yesterday. "It's absolutely vital that the one-subject interview, pursued relentlessly, remains part of the line-up." His very success, however, may already be sounding its death knell.