Just before 6pm last Friday evening, as warm, tired tourists stood in South Kensington, London, wondering what buses to catch and boys gathered to play football on the sun-dried, savannah grasses of Hyde Park, two men, one a revered Dutch architect, the other an energetic Swiss curator, sat down on black foam cubes in the temporary summer pavilion of the Serpentine Gallery and started asking questions.

Not of or about each other but, as it came to later feel, pretty much anyone and anything else. For the next 24 hours, Rem Koolhaas, the Pritzker-prize winning architect of the egg-shaped pavilion above them, and Hans Ulrich Obrist, the director of exhibitions and programmes at the Serpentine, talked a marathon: interviewing artists, writers, comedians, set designers, filmmakers, music-makers, ballet-makers, transport managers and mathematicians, only stopping for occasional breaks and a plate of pasta.

For a spectator who sat and lay (napping only intermittently) through it all, it was a joined physical and mental experience. At 7.11pm (just over an hour in), I experienced a strange pain in my left shoulder as Zaha Hadid, a brilliant architect sheathed in clothes of large black folds, discussed the reinvention of the courtyard. From then on there was hunger (answered by right-on gallery food: spinach quiche and green tea), fatigue (I succumbed to broken sleep after 16 hours), difficulty talking (2-4pm the next day) and nausea.

But these were just the symptoms of a more general condition: the addlement of listening to a monumental conversation. Here are a few of the things that came up in a solid day of high-brow talk: cosmogenic art, surveillance cameras, music in Beijing, Lagos in the 1970s, Ireland in the 1920s, British music in the 1990s, situationism (Parisian and London-based), the Japanese metabolist movement, theosophy, television, the stupidity or otherwise of money, air conditioning on the Underground, commuting from Glasgow, Hackney Wick, the movement of "bads" as opposed to the movement of goods, prime numbers, photography, post offices, bomb sites, Afro-futurism, science fiction and the survival of Arabic culture in Sicily.

Then there was the range in tone. From the touchingly banal -- "Why do you want to learn Chinese?" Because I want to speak to people in China" – to the absurdly abstract. Both Obrist and Koolhaas (for whom The New York Times coined the expression "Rem-speak") are happy at the very heights of conceptual chatter. My unofficial prize for the least comprehensible contribution went to Tom McCarthy, an artist and novelist, for describing a club he runs as "a network, part of which is public, part of which is propositional" and his intention to "turn London into a technologically mediated tomb". He became a champion with the admission: "I don’t know if I’d be comfortable saying something." An honourable mention, though, for Shumon Basar (writer, editor, curator, educator) for saying "an atlas of spatial practice" just before 5:15 in the morning. Other fine terms included "communicational force", "originary" and "irridisibility". Hanif Kureishi, at 8.08pm (two hours in) was the first guest to swear.

None of this would have been possible without Obrist and Koolhaas, who sat, surrounded by books, bottles of Perrier and plates of fruit, while all around them the crowd and even the pavilion itself reflected the passing of the hours. As the audience went from early evening beautiful to late-night drinking nerdy to Saturday gallery-typical, and the balloon that is the pavilion’s roof rose and fell in the heat and the cold, the two men, with their inquiring, continental voices, became the guy-ropes of the conversation, the route of the marathon. They worked as a pair, passing notes, whispering to each other ("Do you have a question, Rem?" "No, almost"), but the curator and the architect talked differently.

First there was Obrist: boyish, untiring and eager under receding curly hair. A pathological interviewer – he has published several books of conversations with artists, historians and writers – he provided the rhythm, the metronomic questions that gave the event even the loosest shape. By asking about London, the conversation’s notional subject, and his own elliptical "How do you see the current moment?", Obrist drew out the only consistent note, which was, perhaps unsurprisingly, one of pessimism. From Brian Eno in the very beginning ("Sometimes I think things are so precarious that everything we do is irrelevant") to Doris Lessing at the very end ("We are a calamitously stupid species"), very few of the interviewees saw the future with too much hope.

Koolhaas, opposite the relaxed, flexible Obrist, was squirming and tense, crossing and unfolding his arms. While Obrist showed no discernible signs of tiredness, as time went on Koolhaas, who spends 300 nights a year in what he calls the "junk space" of hotels and aeroplanes, became increasingly mortal. At 2.12pm on Saturday, just over 20 hours in and faced with Chantal Mouffe, a loopy-voiced Belgian political theorist, he seemed to speak for the audience as well as himself: "We are deeply exhausted… we are scared." With his questions, he was a balding, ageing boxer, looking for the knockout blow. And that was the shape of the conversation, after a gentle inquiry from Obrist would come a bewildering Koolhaas jab, among them: "I see centrality in everything you do", "What is inherently the reason we are stuck?" and my favourite: "I would like you to describe your moment of weakness."

Some of them didn’t come off. For instance this exchange with Ken Loach, the film director:

Koolhaas: "I think one of the strong effects of globalisation is that it means people cannot stay with a particular mentality, or a particular sophistication, they have to adjust all the time. Do you find that?"

Loach: "Er, no, not really, perhaps I should."

Some of them did: like this encounter with Damien Hirst, who with Ant Genn, a guitarist from Pulp, turned up to an improbably full pavilion at 4:20am.