The playwright Simon Gray once called Stephen Fry "the man who wasn't there", after Fry famously did a bid a bunk from Gray's West End play, Cell Mates, in 1995 - intimating that you could talk to Fry for years and never get to know him. How much, then, for my chances of understanding the great polymath in 50 minutes over a pot of tea at the Groucho Club?
But then celebrity interviews are almost always akin to speed dating - and the best you can usually hope for is to form an impression. It helps that Fry is expansively voluble at "home" in Groucho's (he's a founding member of the London club) - his rich, fruity voice audible from the other end of the bar as I enter. He begins by asking whether my name is pronounced in the French manner (such a Stephen Fry thing to do, that; like an interested society hostess), before launching into an extensive anecdote about how scientists have discovered that human sperm is attracted to the scent of lily of the valley.
He's holding court to promote the new series of QI, the BBC2 comedy quiz show he hosts, and he thinks that sperm's attraction to lily of the valley is typical of QI's function of being both amusing and informative. "Alan Davies, one of our regular panellists, can be surreally funny about the idea of giving sperm flowers to smell, while I make the point that there will be people alive in 20 years because of it," says Fry, quickly finding his stride. "The fact is that people have always wondered what it is that drives the sperm towards the ovum and it was always assumed that there would be a receptor on the sperm..."
Well, yes, great, but we must move on. One journalist compared interviewing Fry to listening to the radio, but that's unfair. It's a bit of a performance, sure. And he could talk the hind legs off an armchair, but only if you let him. He's quite happy to take direction and answer your questions.
QI is from the same school of television as Have I Got News for You, and Alan Davies is (sort of) the Paul Merton to Fry's Ian Hislop. Or as Fry puts it, "I become the sort of querulous schoolmaster and he becomes the naughty monkey who doubts the value of all these facts."
On the subject of Have I Got News for You, is it true that Fry was offered the job as Angus Deayton's successor, and turned it down? As he told one reporter who asked him about Deayton, "He takes coke and has slept with a prostitute - but he's a TV presenter for God's sake."
"There is an old saying amongst players of chemin de fer and baccarat, 'Never take over a winning bank'," he says now. "It was a show in a good state and it could only go downhill from there. Also, I thought they dealt with Angus very shoddily. I think if they'd just held on for a couple of weeks the whole thing would have blown over. And I thought Paul [Merton] and Ian [Hislop] were a bit mean to him, and the atmosphere didn't seem very nice to be around.
"I like the fact that there is none of that with us; so far, there haven't been any hissy fits on QI and there's no competitiveness about who's the funniest or who's getting the most airtime."
Fry had originally wanted to be a regular guest on QI, not its host. "Perhaps I have a completely misplaced view of my own acting abilities, but I think the more you become a personality on television, the less likely you are to get acting roles. You know, you don't offer Jonathan Ross parts in movies."
That Fry did accept the presenter's role says a lot about his thirst for knowledge (or, as he puts it, "my own rather nerdy love of facts"), the sort of thirst that had him, as a child, allegedly learning the whole of The Guinness Book of Records. "I didn't learn the whole thing, that really would be weird," he says, "but I did learn rather large sections of it - particularly things like the longest attacks of hiccups, or sneezing fits. I had a big poster of Daniel Lambert, who was the Fat Man of Stamford... fat leaking out over his shoes. I obviously knew what I was going to become."
The love of facts, he says, stems from his father, the physicist and inventor Alan Fry, and he offers this rather sweet Boys Own picture of life with the Fry family in 1960s Norfolk (he has an older brother, Roger, and a younger sister, Jo). "We might just be talking and a word would be used such as 'bivouac' and he'd say 'I wonder where that comes from' and he would always go and look it up and we would crowd round the dictionary and say 'oh look,' it comes from that'."
It's perhaps not surprising, then, that the next acting role in which we'll see Fry (sometime around Christmas) is that of the great 19th-century educationalist Thomas Arnold in ITV's new version of Tom Brown's Schooldays. What? ITV, the home of Bad Girls and Footballers' Wives, is filming a book by the archetypal muscular Christian about the birth of the modern public school?
"I know what you're getting at", says Fry. "We live in a world where people are constantly using the dreadful word 'relevant'. It's as relevant as any story could be - bullying is one of the hot-button issues of our day, and Flashman is the archetypal bully."
Fry says it was his "duty" to research Arnold thoroughly, but when it came to playing the Eminent Victorian ("more of a Hanoverian, really"), he had to wear that knowledge lightly. "You can't play somebody who happens to be simultaneously writing the complete apparatus criticus on Thucydides, penning five sermons a week and who was also Regus Professor of Oxford while being headmaster of Rugby. You can't wear that on your face."
I had asked him about researching Arnold because I was wondering about a fairly common perception of Fry, that he sometimes tends, whether unwittingly or not, to play himself. It's a fear that has certainly been voiced by some Sherlock Holmes scholars on learning of Fry's intention of playing the great detective for TV, with Hugh Laurie as Dr Watson. But first there is a more immediate obstacle to this particular role.
"I've given myself six months to lose 40 stone", he jokes. "I think I ought to. It's very noticeable that words like 'lean' and 'cadaverous' are used in Holmes but words like 'lard arse' are not. Not once does Conan Doyle say, 'Holmes wobbled over to his chair and sat down and stuck a pipe into one of his chins', Fry continues, doubling up with laughter. Collapse, as they say, of the stout party.
With Fry playing Arthur Conan Doyle's sleuth for ITV, and Rupert Everett donning the deerstalker for the BBC, this is surely the first time we've had two gay Holmeses. "I don't know about the private life of William Gillette, the first movie Holmes, and I can't believe that Basil Rathbone was camp," says Fry, greatly amused by the thought.
Fry himself has been openly gay for ages, although for many years he was famously celibate, claiming disgust at the idea of "rubbing the wet slimy bits" of his body over other people. Now, though, he's been with his partner, Daniel Cohen, for "eight years and two months", and, he says, learning to be "less obsessed with work and achievement".
"There's also this lovely thing, the first person plural, that for 15 years I never really knew. People would ask me, 'Have you seen that film?', and I used to say, 'Yes, I saw that'. Now it's so nice to say, 'Yes, we saw that'. It's a very pleasing, deep human need just to say 'we'. 'We like that, don't we, dear?', that sort of thing."
Fry and Cohen divide their time between Norfolk and Hampstead, and he has talked of great drinking binges (he claims to have given up drugs) in the capital. "That's true when I've been writing up in Norfolk," he says. "Because I write in chaste purity, and, indeed, almost in physical ecstasy, because I barely eat, I get up very early, and all I really have is coffee and cigarettes. I'll do that for months and months, and when I've finished I'll come down to London and have large, spacious evenings..."
This is a man who socialises with Prince Charles and Peter Mandelson, as well as the usual roster of comedian friends. Who does he go "larging" with in London?
"Hugh Laurie, Ben Elton... But surely you're above wanting to know about my showbiz pals?" Not at all, I assure him, but he's moved on... "I have a particular fondness for restaurateurs. Some of my best friends are cooks and chefs - they know how to have a good evening."
The young, however, he feels have lost the knack of entertaining. "Kids don't seem to have the energy to do anything remarkable or imaginative. In the 1920s, if people had a party, they had an extraordinary theme. You know, the Sitwells would have a 'paradox party' where you had to come as a new paradox. But now invitations come covered in banners like a bad website. People can't be bothered to throw a party without getting it sponsored by vodka manufacturers or ghastly luxury goods companies such as Luis Vuitton. It's so squalid and dispiriting."
All of which makes Fry sounds like a grumpy old man, something he denies. "I was asked to be on that Grumpy Old Men programme and I refused because I said, 'If I go on, I will be grumpy about grumpy old men.' I think there's nothing more pathetic than people moaning about mobile phones."
In fact, one consistent claim by Fry over the years is his absolute dread of becoming bourgeois. "There's a line in Howards End... Leonard Bast - he's 17 and a clerk in the City - and Helen's looking at him and she uses the damning phrase, 'He'd given up the glory of the animal for a tail coat and a set of ideas'. I'd hate that. I like to wake up each morning and not know what I think, that I may reinvent myself in some way."
I cannot ask what the next transformation might be, for our time is up and he's off, leaving the impression of someone who both genuinely enjoys his intellect and can wield it like a defensive forcefield. Either way, he seems jolly happy, and, to use Fry's own words when describing scientists discovering sperm's attraction to lily of the valley, he is "oddly endearing".
'QI' returns to BBC2 on Wednesday at 10pm