Ask Armando Iannucci to describe his show and he'll tell you what it's not. It's not a stand-up show. He doesn't do jokes. (It has been described by one wag as a story-telling peculiarama.) "The idea is that you'll go away from it having felt you've been to a funny show - as funny as a stand-up show - but you don't feel you've been to a stand-up show: like I come on, do my 100 jokes, I come off."

It's basically a question and answer format. What sort of questions? "Well, the audience want me to be funny, so those are the questions they ask. Like what do I think of Tony Blair? Every night goes slightly differently, but I bank on the same sort of questions coming up. So it's fairly structured in that I know what I'm going to say at the beginning and I know what I'm going to say at the end, and I know how I'm going to divide it." In fact, Iannucci reckons only 20 per cent of the show is done "on the hoof", because although he describes himself as a writer/performer, the emphasis is very much on the writer. Although fans of the wacky and surreal will recognise the puckish host (and only begetter) of Friday Night Armistice, his particular brand of off-beat humour - if not his face - is familiar to the broadest possible TV audiences as producer and co-writer with Steve Coogan and Patrick Marber of Knowing Me, Knowing You .. . With Alan Par, which won Best New Comedy series at the 1995 British Comedy Awards. The character of Alan Partridge began on radio, where Iannucci was already a producer, for a show called On The Hour. "I wanted to do this news parody. So I was looking for people who had comic minds, who could do characters but who weren't like actors, who could improvise and could contribute. Chris Morris was the presenter and Alan Partridge was the sports reporter. I had heard Chris doing news parodies on Radio London and I knew about Steve and when we did the pilot, it just all gelled."

With Coogan, the writer/performer split is the other way around. "Steve is a better improviser than he is a writer, so when we're writing Partridge, we talk to each other and he has got to start behaving like the character before the words come out."

BUT words are what kick-start Armando Iannuci's imagination, indeed the return to performance is entirely down to his writing. Last year a collection of his anarchic bi-weekly Guardian columns was published as Fact And Fancies (some of which were broadcast on BBC Radio 4 last Christmas). These pieces are classic Iannucci territory: step by step everything seems to make sense .. . but it's not straight-forward. An example is the connection he makes between Gerry Adams and Moira Stewart that sometimes comes up in the show. ("Take away the beard, the glasses, the bulbous eyes, the pursed lips .. .") It's what he describes as deceptive logic. "I'm not very good at realism. And I like the idea of something that starts off normal and you're led into it and before you realise it, you're somewhere you never expected to end up. All the steps have been perfectly logical. So you're not troubled by how you got there because you just didn't quite expect to get there. I like stuff that starts off conventionally, ends up being completely bizarre but still has a kind of sense about it." Iannucci's ability to go off on a jag is intoxicating both to the audience and to him. The formal constraints of a book reading - the inevitable side order to nineties' publishing - didn't really appeal. "I felt guilty about asking people to pay money just to watch me read. So I wrote some extra stuff and this took over and I was reading less and less from the book." He not only enjoyed doing it, but realised how much he'd missed it. "I have had a strange career path. I imagined I would struggle for a while as a performer and then gradually something would happen, but it went a different way. I ended up producing stuff and the programmes became more famous than the people in them. I felt I'd missed out on a good few years of live performance so I'm almost out of sync now and I want to go back and do that bit and fill that bit of experience in. Working as a performer really sharpens you. It's such a solid experience you can learn so much from working live. In television you make a mistake, reset the camera and do it again. Live you can't say: "I'm sorry about that. I'll be ready in a few minutes, just talk among yourselves and I'll re-set." Armando Iannucci owes his exotic name to his parentage (both sides Italian) but his accent and his humour to Scotland, where he was born 33 years ago. ("Humour isn't genetic. I don't know what Italian humour is.") He believes that both Irish and Scottish humour have developed "as a way of detaching yourself from English culture".

"English culture is about realism and narrative and neatness. And the Celtic humour is more absurd and fanciful. It's about revelling in things. I mean look at Joyce." And for all his man-of-the-people persona, Iannucci knows his literature. A clever lad, he spent six years at Oxford reading English, culminating in a PhD on Paradise Lost which he never finished, though by coincidence he now lives a hundred yards away from where Milton once lived. His other influences include Dickens ("grotesque"), Swift and Gerard Manley Hopkins ("really weird"). A one-man show at the Edinburgh Festival led to the offer of a job with Radio Scotland. Even though his act was very visual ("I used to do this mime of Margaret Thatcher being swallowed by a giant pike. I played both parts") radio was where his comedy had its origins. He still has tapes made 20 years ago: The Hitchhiker's Guide to the Galaxy, Radio Active, The Burkiss Way - shows that not only formed his taste in comedy but gave him material for the school concerts which he would always end up compering, "doing impressions of Harold Wilson, stealing lots of jokes and making an act out of that".

And it was all a bit of an act, perhaps even still is, a mask for someone who is happy to describe himself as introverted (though less happy to be called shy). He still finds it very difficult to make small talk and was recently horrified to find himself at a celebrity dinner where guests were expected to discourse on affairs of the day. While this "terribly intelligent and articulate badinage" was going on, Iannucci says he just wanted to get up and run. "I was thinking this is a nightmare. This is meant to stop when you've left university. At long last I'm going to be found out about being inarticulate and an ignoramus. I was always surprised that I was good at maths and physics. I always felt I was the worst one there. I always felt it was a mistake and they'd find me out." His mother, he says, has only recently come to accept that her academic son is never going to present Panorama. "She did think I would do so, since I was in the BBC. For her there was an inevitable transition from silliness to seriousness."

Deceptive logic.

Armando Iannucci performs in Cleere's, Sat and Sun 9.30 p.m.