Philip Norman: The human drama that unfolds in every snatch of overheard conversation

My favourites are pairs of lovers, especially at that early, transitory stage

The other morning, I was waiting in the concourse at London King's Cross – wondering why all mainline stations nowadays have to smell of Cornish pasties – when a hugely tall, long-legged Buddhist monk sat down on the bench beside me, fumbled inside his brown robe and took out a mobile phone.

"Nigel," he said into it in a stricken Australian voice, "I've made a terrible mistake." What do you think the next line was? That the wicked metropolis had somehow tempted him into breaking his sacred vows? That he'd realised he wasn't cut out to be a Buddhist monk after all? Actually, it was: "I've left my suitcase on the Tube. It's going round and round on the Circle Line and I've got no idea how to get it back."

Like most of the human drama we all eavesdrop on daily, this one ends in a question mark. I have no idea whether the poor monk got his case back or whether it continued going around the Circle Line until the security services blew it up. Quite a bang all those prayer beads and holy books would have made. With most people (sensible people, I mean) such conversational snippets are instantly forgotten, if indeed they're ever picked up in the first place. But with me, even the most fragmentary tend to become etched on a mental memory stick and replay themselves over and over.

It's a habit I've had since I was seven or eight, and would overhear, if not witness, cataclysmic rows between my parents about the girlfriend my father was publicly carrying on with at the time. They never seemed to notice I was there, and I seem to have retained that quality of invisibility whenever people are unburdening themselves in earshot. I also have the journalist's ability to hold a conversation while listening to what's being said all around me. Useful enough in my trade, but a curse when I lie awake at night and all those random offcuts of dialogue come flooding back.

To take an example (once again, don't expect a punch-line), here are those two naval officers, a lieutenant and a midshipman, whom I overheard as a 10-year-old while my school was visiting their frigate in Portsmouth Harbour. I can still see the green, seaweedy water, the riveted grey steel and their two starched short-sleeved shirts:

Lieutenant: "Hello, Snotty. What's happened to Callaway?" Midshipman: "Mr Callaway couldn't come, Sir, so he sort of sent me along in loo."

I was brought up in the pub trade, and so spent my early years surrounded by inconsequential conversation – heightened by alcohol's power to stir people to exaggerated modes of speech and unaccustomed musings on life, art or religion. A great but underrated British novelist, Patrick Hamilton, brilliantly depicted these saloon-bar orators, whose talk always ran along the same lines and – so far as I can overhear – still does today: "Landlord! Two more pots of your finest ale, if you please. And a teensy-weensy gin for my Lady Wife ..." "Well, ideally of course, there should be no such thing as racial discrimination ..." "Look, I'm not religious, but it stands to reason. There's got to be Something Up There, right?..." 'Look, it stands to reason! There's only so many musical notes that can be arranged in different patterns, so there's got to come point when every possible tune has been used ..."

Alan Bennett has admitted how many of his funniest lines came from women he overheard talking on buses. His forerunner in this field was Al Read, a hugely popular radio comedian of the 1950s. Travelling on a Lancashire bus to research his monologues, Read noted the following:

1st Woman: "How many children has young Stan given your Ida now?"

2nd Woman: "Four."

1st Woman: "Ooh ... the dirty beast!"

2nd Woman: "Well, they've come to an arrangement now that she'll only sleep with him once a year. And you know, bless him, he does look forward to it."

Now and then, the social eavesdropper is rewarded with self-contained drama. One night, in a crowded, candlelit bistro, I sat beside a man straining every nerve to woo a haughty young woman in matching purple shoes and eyeshadow. To impress her with his wealth, he showed off his Rolex watch. "I've always wanted a watch like that," she said icily. "Darling, it's yours," he mumbled, and handed it over. A few moments later, exactly the same happened with his Mont Blanc pen.

Having still got nowhere, his final gambit was to warm her brandy glass masterfully on the candle flame, not realising that would smear it with soot. He handed her the glass, then plucked up courage to stroke her cheek, leaving a thick black mark that she was completely unaware of. At this point, I could bear no more and got up and left ... so, once again, no resolution.

Going out jogging has added enormously to my collection. No one seems to notice joggers or feel the need to lapse into discreet silence as they go flip-flopping past. Nearly every day, I catch an intriguing glimpse of a life or situation, but always have to leave the punch-line in my panting wake. Take this example only last week, from two teenage boys walking through torrential rain in their shirtsleeves:

"Your Grandad flies all round the world, doesn't he?" "My Dad ... yes." "And he was the President, right?" "Prime Minister." "That's so cool..." Or this, between two elderly street beggars: "I've just seen my favourite member of Cream." "Ginger Baker?" "No, Eric ... Eric ..." Or this from a teenage girl to her friend, whom I expected to know about nothing but pop and Heat magazine: "Leviathan ... Colossus ... wotever..."

My favourites are pairs of lovers, especially at that early, transitory stage when the young woman gazes into her swain's eyes, dumbstruck with fascination at everything he says. One I passed was getting this treatment as he talked about "starting a consultancy for a rubber-derivatives company"; another, while he described every stage in the fermentation of cider.

The curious thing – my poor, absent-minded Buddhist monk apart – is how little good stuff comes from conversation on mobiles. It all seems to be, "We're just pulling into Biggleswade station", or "Can you email the new business plan to me by tomorrow?" or "No worries, mate", or (overheard for real, from inside a cubicle in a cinema Gents'), "I'm on the bog". Like everyone else when annoying ringtones detonate, I do my best not to listen.