In the early Thirties, John Lahr's parents, Bert, the comedian who played the Cowardly Lion in The Wizard of Oz, and Millie, a former Ziegfeld girl, strolled home in full evening dress across New York's Central Park after a night out at the Tavern on the Green. They passed a group of dispossessed men warming themselves over a garbage-can fire and the men waved at them. 'Weren't the men angry at the sight of you?' John Lahr asked his mother years later. 'No,' she replied. 'We gave them hope.'

That same ambiguous mixture of hope and envy with which people regard the famous is one of the underlying themes of Show and Tell, the collection of Lahr's New Yorker profiles published this month. For if, as he suggests in the book, celebrity is democracy's substitute for aristocracy, here is a chance to admire the noble profiles of the lords and ladies of the American arts: Frank Sinatra and Irving Berlin, Bob Hope and Roseanne, Woody Allen and Arthur Miller. There are a couple of foreigners in there, too: a British prince (Eddie Izzard) and a Swedish count (Ingmar Bergman), but the majority are people who have helped to define the way the world sees America.

One New Yorker profile he hasn't included in the collection is that of Kenneth Tynan, the magazine's former theatre critic and one-time theatre critic of The Observer. Instead, Tynan is the subject of Lahr's other book out this year, the long-delayed Diaries of Kenneth Tynan, which he has edited. For years, rumours of the content of the diaries, with all their sado-masochistic elements and tales of sexual experimentation, have floated round publishing houses. Now, in the same year that Tynan's first wife, Elaine Dundy, has published her own memoirs, Life Itself, the diaries will finally arrive. They had been held back for some years by Tynan's late widow, Kathleen Tynan.

'A lot of people thought it was not publishable,' says Lahr of what he was given. 'I thought it was great. Often when people encounter the dark things they think it will darken the memory.' He felt that the reverse was true in that it gave a fuller picture. Readers will have a chance to make their minds up shortly.

As the son of one of America's favourite film creations, Lahr, now 60, had his own early taste of the aristocracy of celebrity. He had been born in Los Angeles at the height of his father's fame but they had left their Coldwater Canyon home for New York after his father had concluded that there were only so many lion parts on offer.

'If you are the son of a star, you are like minor royalty,' says John Lahr over a lunch of salads and iced tea by the (modest) pool of a friend's house in Santa Monica; a bottle of wine sits untouched between us - this is California. 'That's, of course, ridiculous but what it does give you is a sense of infinite possibility.' And he quotes the words of one of his subjects' most famous songs: 'Come fly with me, let's fly, let's fly away,' the anthem for the 'preppy white boy' he saw himself as in the Fifties at a time when the suffix -ville, as in 'coolsville', was added to everything and guys tied their Brooks Brothers ties in Windsor knots.

After that 'preppy' childhood in Manhattan, he went to Yale. A BA at Worcester College, Oxford followed. He has managed to be properly transatlantic ever since, being a drama critic at one time or another for the Village Voice , the New Yorker, the Evergreen Review, British Vogue and New Society where his then colleagues included Julie Burchill and John Berger. He married his first wife, Anthea Mander, in 1965, and their son, Christopher, is now in the film business in London.

Being minor royalty opened doors back then- he quotes S.J. Perelman's saying that 'in the aristocracy of success there are no strangers' - and being the New Yorker 's writer on theatre and popular culture since 1992 has opened the stage door since. It is, he says, the perfect job, although it means that he commutes between a room in a friend's flat in New York and the London home that he shares with Connie Booth, the actress and writer and now psychotherapist, whom he married last year after 15 years together.

Some of his subjects do indeed both show and tell very entertainingly. When Mike Nichols had made Who's Afraid of Virginia Woolf?, with all its adultery and drunkenness, Warner Brothers was worried that the then powerful Catholic Legion of Decency would not approve it. Nichols came up with the solution: 'When the Monsignor sees the picture, Jackie Kennedy will sit behind him. When it's over she will say, "How Jack would have loved it."' Jackie Kennedy did as she was asked, the film received the Legion's blessing and went on to become the first film ever in which all four leading actors were nominated for Oscars.

That story tells us much about the fame that Lahr has so assiduously explored over the years in more than a dozen books, from his portrait of his father, Notes on a Cowardly Lion, in 1969, through Prick Up Your Ears, his 1978 biography of Joe Orton, which was later turned into a Stephen Frears film starring Gary Oldman, which Lahr co-produced. He is in as good a position as anyone to compare the different takes that the British and Americans have on celebrity.

He describes the famous in America as 'the unelected legislators of the culture'. But in Britain, he believes, there is a pre-emptive envy that affects how people exercise their celebrity: 'You don't show what you've got in order not to create envy, whereas in this culture [the United States], it's reversed. You are always measuring the difference between yourself and other people. And it's a way of selling a way of life - these people are the performing workhorses that prove the system works. The envy that famous people stimulate in a community is the gasoline that the culture runs on. In Britain, where there is not so much to win - Britain is the size of Utah - you can't go as far literally and figuratively, celebrity is a kind of metaphor of abundance. There is envy and irritation that is acted out.'

While he believes that celebrity 'keeps the culture from looking seriously at itself', he thinks that the way it mutates individuals is 'incredibly interesting and very important'. And he feels that many of the subjects he explored have been changed, both in the work they produce and in their lives, by the onset of fame.

'What these profiles do is study the degree to which the aggrandising self separates the creative person from his instincts and from the community, and what happens when you are isolated. So many of those famous people who start out hopeful and promising and full of life lose their connection to the community that fed them. The stakes are so much bigger here and it's so endorsed in this culture, while it isn't in England.

'I've had friends who were really successful in England who have wanted to come to America just to enjoy their success because they couldn't buy the car and park it (without someone scratching it.) Here they'll say, "Hey, what a great car!"'

Lahr's New Yorker profiles involve four months of research, 1,500 pages of transcribed interviews, 10,000 words for 900,000 readers. Invariably, his most heat-seeking questions target the childhood of the subject and, almost without exception, there is pain or tragedy to be found there. Whether it is Ingmar Bergman being made to wear a red skirt as a child as a punishment for bedwetting or Mike Nichols being placed as a child with an English family who kissed their own children goodnight but shook hands with him.

Lahr says that he must feel some empathy with his subject but it is clear that he finds some (Woody Allen, Eddie Izzard, Arthur Miller, Wallace Shawn) much more beguiling and empathetic than others (Bob Hope, Frank Sinatra). In fact, Bob Hope is the one profile in the book in which the subject emerges as almost irredeemably unlikeable. So how did Hope get selected?

'I don't admire him as a comedian but as a phenomenon. The guy was smart, he saw a need in the culture and he franchised himself. But who ever heard of a genuine comedian being adopted by the mainstream? The whole point of comedy is to transgress against the culture. He was bad news, a man who not able to be faithful, who was always on the move - what was he running from?'

In his profile of Roseanne, Lahr concludes that, despite the nose job she talks about, 'nothing can erase the evidence of her soul's unease'. Does he believe that he can make such a judgment? 'You certainly can, because if you meet her it's incontrovertible. You can't tell a lot from just meeting a person once, but if you spend three weeks with them and see them in situations where they are not in control, which is rare for a celebrity, you can see it. It's not an injudicious generalisation. If you shake her hand you'd know it.'

But he is full of praise for her: 'up until Roseanne, the phallic aspect of comedy for women had not been allowed. Everyone had to be oblique, whether it was I Love Lucy or Fanny Brice, the aggression had to be sideways. Roseanne talks about that great line she always ended her stand-up with "People say I should be more feminine - well, they can suck my dick." That's just hugely funny and it tells you a lot about her.'

Some have found that the offer of a profile was one they could refuse. Mel Brooks, fresh from his triumph with the musical version of The Producers , is one. 'He said, "John, I've been reading Show and Tell, it's really terrific but I don't want you doing to me what you did to all those people. I don't want to be looked at in that way." He didn't want to be scrutinised psychologically - fair enough.'

Lahr is currently working on a profile of Judi Dench and on the second part of the biography of Tennessee Williams, the baton having been passed to him by the late Lyle Leverich who wrote the first part.

Since the book is called Show and Tell it is perhaps only fair that Lahr should do some showing and telling himself. This he does in the form of the two profiles of his parents; his mother, Millie, is the only woman apart from Roseanne among the 15 profiles. Not surprisingly, hers is the most affecting, sprinkled as it is with the kind of elegant throwaway observations for which Lahr has won his own celebrity: 'Millie never married after Dad died but she had offers. "I never wanted to cook for a man again," Millie said; she had never willingly applied heat to food in any case.' Lahr's father's original name was Lahreim but when Lahr junior used it on one occasion, his mother chided him. 'Face it, Mom,' he told her, 'you were married to one of the great Jewish entertainers.' 'John,' she replied, 'your father was not Jewish. He was a star.'

In the Mike Nichols profile, Lahr's own favourite, he concludes by telling Nichols that he (Lahr) does well 'with the fundamentally inconsolable', a state of mind with which, he says, both his parents lived.

Kenneth Tynan was another fundamentally inconsolable person, he believes. The diaries, he says, show Tynan on an 'almost tragic trajectory'.

'They are amazing and incredibly sad - telling the story of someone who has lived in the slipstream of fame and finds himself with nothing. He wants to speak because he can speak so eloquently and fabulously but he understands more than anybody that esteem is awarded on evidence and he can't produce it. It's like seeing a man in the middle of a railroad track watching a train come down the line and just not moving and getting whacked.'

Tynan was, says Lahr, 'a man of appetite - what he was always seeking in every sphere was solace'. His intensity made him a great critic, but despite the esteem in which his criticism was held, 'there was a terrible emptiness he was trying to fill'.

As for himself, 'I'm definitely consolable,' he says, as we say our goodbyes. I drive off back into the city that has more of the 'aristocracy of democracy' per square tiled foot than anywhere in the world and, it sometimes seems, almost as many of the dispossessed, the people who watch as the folks in their evening finery stroll effortlessly into premieres - and wave at them.

• Show and Tell: New Yorker Profiles is published on 3 September (Bloomsbury £20); and The Tynan Diaries on 22 October (Bloomsbury £25). To order copies for £17 and £20 respectively call the Observer Books Service on 0870 066 7989