A speaker. A speech. A microphone. A bare stage. And - no expense spared here - a glass of water. As an evening's entertainment, it does not sound like much: Bruce Springsteen and his E Street extravaganza this is not. And yet when Malcolm Gladwell, a Manhattan-based journalist, turned up last winter to do a monologue at the Lyceum, a West End theatre that has hosted Led Zeppelin and is now home to the Lion King musical, he filled it - twice. Despite bitter November temperatures, long queues formed and the first show had to be delayed by half an hour to squeeze in as many punters as possible. All 4,000 tickets, at up to £25 a head, sold out.

What were they getting for their money? Gladwell does not do stand-up, is not in exclusive possession of the Lord's wisdom and cannot tell you how to make millions from buy-to-let. A small, skinny, former business reporter with a towering afro and hands that flutter about as if evading an invisible butterfly net, Gladwell likes to address such pressing issues as the quest for the perfect pasta sauce (Google the video: it is brilliant).

That winter evening he talked for just over an hour - about plane crashes. There was no video, no Q&A, and his material was hardly roll-in-the-aisles stuff. Yet Gladwell pulled it off - so much so that he is back here this week on a mini-tour, playing just enough dates - from Liverpool to Brighton - to fill the back of a concert T-shirt. "There are hundreds of people like me," he says. A decade ago, a scientist, a policy wonk or a writer with a big idea would publish it in books and articles; now they also take to the road and talk about it - in lectures, debates and book readings. The talk is turning into a performing art; the intellectual is becoming a stage act.

"Talks are stand-up without the jokes," says David Johnson, Gladwell's tour producer (now there is a sign of changed times: a journalist with a tour manager). "You get to a certain age and you don't want to get drunk and be deafened by some rock band or heckle some bad comedian. You'd rather be informed instead," he says. Which is presumably why a fortnight ago, the Nobel-winning economist Paul Krugman packed out the London School of Economics for three nights running. Full houses saw a middle-aged man with a greying beard and a blue suit jovially bringing up lecture slides that showed how the current financial crisis has left the world economy in a deep hole. It was the PowerPoint Presentation of Doom, leavened with a fair few jokes - and it went down a storm.

That format - light humour and serious talk, delivered live by a big name - can be caught across the country at regional theatres and literary festivals. This week, Barnstaple residents can spend an evening with Paddy Ashdown and Sir David Frost will visit Ludlow to talk about his life and times. Both appearances will be put on by Clive Conway, who also handles Baroness Shirley Williams, Benjamin Zephaniah and Ann Widdecombe. "It's a chance for people to see someone they would normally see only on the TV, a charismatic and inspiring personality," he says. "They come away entertained but also slightly informed."

Even the highbrows now have established performing names, guaranteed to draw a good crowd. Lectures by Slavoj Zizek - the celebrity Leninist who resembles a cross between a giant bear and Latka from the sitcom Taxi - sell out far faster than any of his philosophy books. And for those who prefer their politics served with more earnestness and less ideology, there is No Logo author Naomi Klein, a warmer, more inclusive speaker whose performances can sound as if she is thinking out loud.

But perhaps the biggest talk event happens next month, when an American organisation called TED holds a four-day conference in Oxford. Up to 60 lecturers will speak for precisely 18 minutes each on subjects ranging from the eastern European mafia to whether solar-powered aeroplanes will ever (ahem) take off. Tickets are going for £2,750 each (although the hard-up get a £1,225 concessionary rate) and only 30 of the 700 are left.

As such prices indicate, talks can make good business. Conway started his lecture firm from scratch in 2001; now he mounts between 250 and 300 evenings a year. David Johnson has a background as a comedy producer, but after promoting Gladwell he wants to do a show with the poet laureate, Carol-Ann Duffy ("She's got the makings of a superstar"). His epiphany came in the winter of 2002 when putting on a series of spoken-word performances in London by the US film-maker Michael Moore. The show was a mix of polemic against the imminent war on Iraq and broad jokes such as Moore using a leaf-blower to gust his underpants around the stage. Most of all, it was a hit: 800 seats a performance at up to £30 each, eight shows a week, a five-week run - and the whole lot sold out. "That showed me there was a huge public appetite for serious subjects," says Johnson.

Gladwell, Moore, Krugman: you'll have noticed the predominance of US names (Klein is Canadian). This is surely no accident, because what this shift also suggests is that the British intelligentsia, like so many other sectors, is becoming more American.

In the US republic of letters, big names employ speaking agents to schedule their lecture engagements and can expect thousands per show. Gladwell, for instance, has spent much of his spring not writing, but talking: doing lectures, charity fundraisers and corporate events. He has assembled a repertoire of 12 speeches, and has memorised half a dozen of them, each about 45-minutes long. But wouldn't Gladwell's fans be happier if he delivered a new book or 10,000-word feature rather than a talk? "One of my musician friends has a 16-year-old daughter who never buys a CD but goes to concerts the entire time. Me, I'd prefer the recording," he says. "But it's so easy to get the MP3 or the printed thing nowadays that the scarcity value is attached to the author or the musician, rather than what they produce."

That comparison with the pop industry is a telling one. The rock album is no longer a money-spinner; gigs are where any cash gets made. Book sales in the UK are flat rather than falling (hardbacks are not as easy to pirate as MP3s - yet) but even optimists describe the market as mature. Which leaves publishers and authors looking around for other ways to whip up business, even if that means focusing less on books. "It used to be that when a new title came out, you sent out copies to the reviewers and did a few bookshop signings and that was it," says Caroline Priday of Princeton University Press. "Now you're not marketing the book so much as the author, looking for events and publicity for them to do all year round."

The author-as-performer is not a new conjugation. Charles Dickens and Oscar Wilde were accomplished public speakers, and by the 1920s money-spinning author appearances were sufficiently well established that the newly bestsellered Joseph Conrad gave it a go (with disastrous results: the Polish author's English was so thickly accented that even his private secretary struggled to understand him, and customers were not happy at paying for incomprehension, however literary). But the talk is no longer a one-off. It is a staple entertainment; an evening lecture or reading has become a social alternative to the cinema. There is just one problem with this: no one ever went into historical research to play Wembley.

Some do rise to the challenge. Amit Chaudhuri produces both novels and music and has successfully experimented with performances where he showcases both. The comedian Robin Ince is doing shows where jokes are woven around explanations of string theory. Still, as the job title suggests, writers write rather than necessarily talk. Some have enough difficulty socialising.

For those without such troubles, and a degree of fame, decent money awaits. General Sir Mike Jackson, former chief of the British army, is at a posh hotel in Hampshire at the end of this month talking about his life; at £55 each (including lunch), all the tickets have already gone and the waiting list is over-subscribed too. People who have stumped up to be there make a far easier audience than a Paxman or a Dimbleby. "TV interviews are a form of mortal combat - they generate heat, but not light," Jackson says. "This forum allows for more mutual comprehension."

It also does not hurt that one in five of those listening buys the speaker's autobiography or latest book. "This sort of audience just wants to sit in the presence of a celebrity author and receive his or her wisdom," says Peter McDonald, an academic at St Hugh's, Oxford. "The audience wants the author to be a secular sage."

But mark you, not too mystical a sage. Because writers doing talks usually have to sacrifice subtlety and complexity. In print one can wield stats, deploy graphs and take any amount of detours. Try such fanciness in public speaking, and you risk losing your audience. Ultimately, a reader may be challenged, but an audience must be engaged.

Last week, I went to a debate on MPs' expenses in Kensington, west London. Featuring a fine panel of historians and journalists, it was staged by Intelligence Squared, which claims to put on "intellectual blood sports". "People want to get out of their houses and away from their screens," Hannah Kaye, one of the company's debate organisers, had told me.

At the back of the hall were the couples on dates, fresh from the bar and ready to catch a couple of hours of talk before heading off for supper. What they wanted was party apparatchiks and duck islands, and regular mentions of either were greeted with rousing jeers. Up in the gallery, we were most definitely being played to - until, that is, the discussion got too serious. Then the room temperature dropped: London Evening Standards were leafed through, and a lady began a furious game of Snake on her mobile.

Recklessly, one speaker launched into a taxonomy of the alternative vote, the list system and other details of proportional representation. On the very last row, a woman began stroking her partner's shaved head. Whether it was meant romantically, or to assuage pain, was not clear.