Earlier this month, Paul Theroux appeared at the Singapore Writers Festival, where he gave a lecture on his extensive travels. He also took part in a panel discussion about whether foreign aid undermines recovery in conflict zones. His attitude to this debate can best be summed up in the first line of a 2005 piece he wrote for The New York Times: "There are probably more annoying things than being hectored about African development by a wealthy Irish rock star in a cowboy hat, but I can't think of one at the moment."

And he gave a Sunday afternoon talk at the National Museum of Singapore, which was followed by a screening of Saint Jack, the 1979 film based on his "infamous" - as the programme notes put it - novel. Film and book had both been banned for several decades by the Lion City's government. Now, there they were, revered cultural objects and as much a testimony to Singapore's transformation through the ages as the nearby permanent exhibition on its previous 700 years.

A few days later, on a grey Hong Kong afternoon, he's still marvelling at the unlikeliness of it. During the late 1960s, when he lectured in English literature at what was then the University of Singapore, he could never, he says, have imagined any sort of literary festival in the place.

"Travelling in Singapore and China now," he says, "you feel like Rip Van Winkle. You come back and everything's changed. No one knows you. They take the living arrangements for granted. And you think: I don't recognise this place."

He's in an armchair, having a beer in the Sky Lounge on the 49th floor of The Upper House, in Admiralty. At 73, he doesn't look much older than the owlish, bespectacled American who's been gazing, level-eyed, at readers from book covers for 45 years. The following evening, he's due to address the Royal Geographical Society's annual dinner, the first time he's spoken at an RGS function here and it's a sold-out event. As the society's website puts it, "Paul Theroux virtually invented the modern travel narrative by recounting his grand tours by rail through Asia, Europe and the Americas."

So it's odd to hear this global traveller say, emphatically, "I feel lost in Hong Kong. I don't know how you'd write about it, it's impenetrable. There's so much of it … I don't mean writing about the restaurants and hotels, I mean about the city itself. You'd have to live here to do that."

Which is, surely, a little literary joke, because Theroux has written about Hong Kong. Just before the 1997 handover, he wrote a novel called Kowloon Tong, which did not find favour with the South China Morning Post's reviewer, now its arts editor, Kevin Kwong, who viewed it as cashing in on current events and plaintively asked, "Are the streets crawling with Chinese and Filipino call girls? Why do such a disservice to the place that is my home?" (Judging by the current stereotypes luridly being trotted out by the British media in the wake of the Rurik Jutting case, however, this is an ongoing lament.)

Around the same time, in May 1997, Theroux wrote a long piece for The New Yorker magazine titled "Ghost Stories". The subhead was: "Will the handover mark a new era of repression? Or will it merely make a lot of people very, very rich?" It contains a few breathtaking assertions ("Like chinky-chonk, gweilo has become a term of affection"), some unexpected echoes (a news story of a man running amok, setting fire to umbrellas) and at least one historical irony. When the chief-executive-in-waiting, Tung Chee-hwa, refers in hero-worshipping terms to Lee Kuan Yew, the man who created modern Singapore, Theroux writes that Lee "has so effectively gagged Singapore's press that the place is now paranoid and barren without any debate, without one word of dissent".

The person who comes out best is Martin Lee Chu-ming, co-founder of the Democratic Party, followed, a long-ish way behind, by then-governor Chris Patten. Generally, the British authorities, the Beijing authorities and American businessmen are treated with equal scorn. Theroux writes as he sees and - especially - as he hears, of which more later. Kowloon Tong was a satire of the departing British, he always says, so naturally the characters were going to sound racist.

Theroux's constantly surprised when people think he's crotchety. ("Mr Grump Goes to Hong Kong" was the Los Angeles Times' headline when it ran a piece on Kowloon Tong.) He insists he's a cheerful traveller and it's true that, on a damp evening, despite occasional sneezing and slight deafness from a cold, he couldn't be a more obliging or interesting companion. I've read a fair bit of his writing and it's a strange process interviewing someone whose head you've been wandering around in for years, a vivid place littered with little word souvenirs for your knapsack. (That "owlish" several paragraphs back, for instance, is pure Theroux; he uses it about himself.) But it's almost 40 years since The Great Railway Bazaar and even if you think you recognise the landscape, you're not entirely sure.

There's so much of it, nowadays, that it can feel impenetrable.

"I'M DEEPLY CURIOUS ABOUT Occupy," says Theroux, which is good, because I'm curious to know what he makes of it. How would Theroux shape such an event? In Riding the Iron Rooster, his 1988 book on travelling by train through China, he writes about a student demonstration in Beijing: "I was persuaded that the students had demonstrated on their own initiative. Their grievances were genuine but muddled." He subsequently interviews some in a park (he finds them goofy but loveable) while they're skating on what turns out to be very thin ice.

As it happens, you can see a corner of Occupy from the Sky Lounge and he stands at the window, staring down. After a while, he asks, "Why is there a crane in the middle?", and I realise he's gazing at an adjacent building site, an easy mistake to make because the floodlit pit (with crane) looks a good deal more impressive than the carbuncular tents huddled in the dark drizzle next to it. Readjusting expectations having been a central part of his wandering life, however, he simply says, "Oh I see."

"I don't know what I think of Occupy," he remarks, later. "In 1997, I felt there was going to be trouble. Why was Hong Kong handed back to China? Why didn't it become a republic?"

Well, once the 1898 New Territories lease ran out … "China issued railway bonds at the same time [in 1911] but they didn't recognise them," he replies. "There was a court case about it. People who'd bought those bonds were living in the States and wanted payment and the Chinese said, 'Nothing to do with us'."

(I looked this up online later and found a legal analysis of the Huguang Railway Bonds Case by one E. Theroux. That would be Eugene, Paul's eldest brother.)

"In the 1960s, I was an Occupier," he adds. "Against nuclear testing, the Vietnam war. It was a number of years before protests against Vietnam were accepted as a just cause. People have no notion of history. It's very hard to be a demonstrator until you get critical mass. China's history is a history of catastrophe, disasters, sudden shifts, most of them unpredictable."

He recalls a moment in Riding the Iron Rooster when he meets a fellow American in Shanghai and asks what's going to happen next; the guy shrugs, waves at nearby workers and cranes and says, "We didn't know this was going to happen". What happened next, of course, was Tiananmen, which Theroux sees as vindication of his optimistic-traveller-pessimistic-viewpoint tone in the book.

"People thought I was being negative but I saw students being pushed around in the provinces. I got extremely bad reviews, really bad, for Rooster. It doesn't matter." A pause, a sudden shift. "But I was so angry. I hate reading bad reviews. Reviews are written by self-interested people."

Such reversals happen several times during the conversation; indeed, the exploration he most seems to enjoy is his tendency to be contradictory. Critics often remark that he blurs the line between fact and fiction but he evidently likes to voice, write and experience alternatives. An early indication of this is when he lectures his way round Asia in The Great Railway Bazaar; at the beginning, he is arguing for one aspect of American literature and by the end, defending its opposite. A later example is his second marriage, to Sheila Donnelly, who's Hawaiian-Chinese and runs a luxury-travel PR firm in Honolulu, promoting the antithesis of any journey he'd make in his books.

More infamous manifestations, however, are My Other Life (in which a travel writer named Paul Theroux lives a what-if existence) and Sir Vidia's Shadow: A Friendship Across Five Continents, in which he comprehensively and, it seems at times, compulsively eviscerates V.S. Naipaul, whom he once worshipped. There's a particularly memorable moment when he kyboshes Naipaul's chances of winning the Booker Prize and one of his fellow judges comments, in bewilderment, "I thought he was your friend." This is before the friendship breaks up.

"I have spilkus," he says, of his wanderlust. Spilkus? "It means 'agitated', 'got to go'." But maybe the same word applies as much to his opinions as to his limbs. At the RGS dinner the following night, according to various guests, he will meander at some contrarian length about truth, Harry Potter, literature and the motivation to travel, but hardly at all about his actual journeys.

Anyway, he's now tackling America's Deep South, a perfect opportunity to showcase anomalies in the land of apparent democracy. He's working his way through an appropriate reading list. As his fans will know, he likes to travel physically alone while keeping mental company with a throng of authors.

"From Singapore to here, I read To Kill a Mockingbird," he says. "I'm 73 and I'd never read it before. I didn't like it too much. It's all over the place and [spoiler alert] it's a bust. The father doesn't even get the guy off."

Maybe he's too old to read it for the first time, an opinion to which Theroux graciously agrees.

"And the book's too old. I've been reading William Faulkner, Erskine Caldwell, Flannery O'Connor - they're all too obsessed with the phantasmagoria of the South. My feeling is that Southern writers are invested in the bizarre aspects because they can't bear to write about the day-to-day miseries."

In the final chapter of his latest travel book, The Last Train to Zona Verde, which recounts a spectacularly awful African trip (and in which he fully spells out his anti-aid-agency agenda), he signals his intention to look closer to home next time. "I'm not learning anything, I'm going from one miserable place to another and I think, 'What am I doing here? Why not write about the South, the old back roads of Mississippi?'"

In 2012, therefore, just before Barack Obama was re-elected, he got into a car and started driving. How can he meet anyone that way?

"I went to churches, football games, gun shows. But travelling in the South is like travelling in China - you need an introduction."

We argue about this for a while (I've never encountered such a social necessity and wonder if it's a 1980s Rooster-relic) and then he says, "In the United States, we don't have agreed-upon rules of behaviour."

Last year, for example, he says, he was late for a Southern appointment. "And the woman, who was black, began yelling. She said, 'I know why you're late! It's white privilege!' And as she was talking, her husband, who was white, came in and began eating an apple in front of me. I said, 'You think I'm late because I'm white but I'm not presuming on you. I've travelled the world and no one has ever eaten in front of me without offering me something.' Then I left."

He smiles. "And now I have something to write."

Last month, a New York Times critic, reviewing Mr Bones, his recent book of short stories, referred at length to Theroux's "perversely and provocatively old-fashioned" way of conveying the speech of other cultures. (This was also an issue with Kowloon Tong, although I think the Northern Irish transliteration in his 1983 book, The Kingdom by the Sea, is spot on, as are his observations.) When I, gingerly, mention Southern accents - wondering how the apple-eating scene will read on the page - he immediately says, "I read that Times piece. I quote people all the time! It's the very gist and pith of writing! I spend half my life talking to myself thinking, 'How can I render that?'"

Softly, he does a little Southern-speak practice: "What the heel? What the hy-all? What the heyl?"

He says, "I gave a talk at the University of Michigan in April, and I slipped into a funny accent - I was talking about Singapore - and the temperature went waaay down."

I've read some of the Mr Bones stories and the first-person accent is so similar to Theroux's travel books that I seriously wondered, after reading one tale, if he'd been abused as a child at school in Massachusetts. But when asked, he looks horrified.

"I was an altar boy for three years and no priest ever made an untoward gesture, never made the slightest advance on me." Then he recites the Confiteor, in Latin - word perfect.

These days, of course, the Theroux name carries different connotations. He laughs.

"I was in Vicksburg, Mississippi, sitting eating at a round table with eight or nine other people. They asked me what I did. Yes, they all said they loved reading, but no one had a clue. So I said, 'Well you might have heard of my nephew, he's going to marry Jennifer Aniston.' Everyone knew him."

That would be Theroux, Justin (actor and screenwriter), son of above-mentioned Theroux, Eugene (lawyer). But I was thinking of his sons, Theroux, Marcel (novelist) and Theroux, Louis (documentary maker).

"I met a guy," their father remarks, "who said to me, 'I love Louis' programmes, what do you do?' I said I'd written a thing or two."

Does that give him a pang?

"If you had a dog that walked on its hind legs wouldn't you be pleased?" Such a bizarre reference to Dr Johnson's opinion on the matter of women preaching (Theroux rattles off the rest of the quote: "It is not done well but you are surprised to find it done at all") rather makes you wonder what's going on behind those words. Louis, in a 2007 interview in Britain's The Guardian newspaper about why his documentaries focused on such peculiar people (neo-Nazis, survivalists, porn stars), said, "My dad, being a complex character who inhabits a number of different personas, has made it comfortable for me to be around those people."

When I ask Theroux Snr if he tracks his sons' interviews, he says he does; but he means the ones in which they're doing the interviewing.

"I'm so proud of both of them. I have this privilege of seeing my children talking to strangers in foreign countries. Louis had a programme on South Africa, he was talking to Eugene Terre'Blanche [the late white supremacist leader]." He switches, with sudden viciousness, into an accurate Afrikaner accent: "Don't get cocky with me! Don't get cocky with me! I thought, 'Jesus, isn't this great, seeing him being threatened.'"

The very nature of travel is that people move on to new places. At events with his wife, Theroux remarks, unprompted, he's often called "Mr Donnelly".

"Humility is good for the soul," he adds, and now he's surely joking. "I go into restaurants with Louis and the maitre d' says, 'Louis, Louis, come this way'. And that's all right. I follow him. I take the path of least resistance."