· I've been the political adviser on a BBC thriller serial due to go out next year. It's called State of Play, and it's written by Paul Abbott, of Clocking Off and Linda Green, and I can promise you it's a lot more exciting than that endless thing about New Labour, The Project. For one thing, it's only partly about politics; there's lots of sex and murder as well.

Anyhow, they thought it might be a fun prank to give me a part, even though I am to acting what John Prescott is to catwalk modelling. So we found the perfect role: it's a middle-aged political reporter who asks the main character, an up and coming MP, if he's going to resign. It's a slightly important moment in the plot - the MP suddenly realises that things are worse than he thought - but that might not stop it ending up on the cutting room floor.

The fact that I had only one line did not mean, I felt, that I couldn't write a pretentious diary about my day filming. If Alec Guinness and Richard E Grant can, why not me?

Monday December 23: The call sheet arrives. All seven other characters needed on the day are being picked up by car. My entry says: "O/T" which presumably means own transport, or on the tube, or, possibly "over the top".

Monday January 6, 10.15am: Greatly mollified after struggling from South Harrow tube, where I am fined £1 for crossing some imaginary London Transport zone boundary, to be shown by a solicitous young woman to my own private trailer! Or at least one-third of a trailer, with my character's name - Hugh Priestner - on the door, a couch, possibly adaptable for casting purposes, a bathroom, and a stock of mineral water. No champagne or cocaine, I notice, but another kindly young person brings coffee.

10.45: Notice that my line is different from the one I had mistakenly memorised. Worse, it's nine words instead of five. Can I handle it? I decide to think, to feel my way into the part, to become Hugh Priestner.

11.30: I go to watch some of the filming. The director, David Yates, points out how industrial everything about a film still is. Nothing here is done on the computer screen. There are gantries, cranes, vast lights high above the outdoor set, tracks for the camera to run along, van loads of stuff. Though they are filming only five short scenes, the call sheet records that we will be 85 for lunch.

11.45: David and I discuss my role in depth. I feel that I should be slightly amused by the sight of the MP surrounded by a dozen young hacks. I need a cynical, world-weary note in my voice, and I should hit the word "true" in the line: "Is it true Mr Collins is going to resign?" I think Priestner would call Collins "Stephen" and we decide to try it both ways. This is a lot of thought going into this one line.

12.30: I have lunch with David Morrissey, who's playing Collins. The food is excellent - it has to be if you want to get the best crew. David says that as part of researching his role, he spent a day with Peter Mandelson. I think that is suffering for your art, but he seems to have enjoyed the experience.

1.30: We start my big scene. Half a dozen rehearsals, to get the "blocking" right, followed by half a dozen takes from one angle, and a few more from the opposite point of view. I get chatting to the extras, who are extremely friendly, given that I'm an amateur, stealing their work. The problem at first is that they're too nice; as reporters and snappers they would be vicious, pushing and jostling and yelling at the MP. But by the time we do the takes they are razzed up and almost as violent as real journalists.

2.00: My performance is causing anxiety. David Yates thinks I'm too "twinkly". I explain that Priestner has seen scores of resignations; he finds this one amusing. David says he needs serious.

2.05: After another take he explains that I've gone over the top, giving the line too much wellie. It is perhaps a definition of a lousy actor that he can actually ham up a mere nine words.

2.10: Thumbs up from David.

2.20: Thumbs down. My line was too muttered, lost in the clamour.

3.00: Finally we all get it right, and I go back to my cosy mobile dressing room, where another kindly lady arrives with a contract sheet. I am to be paid! No wonder the BBC is in trouble if they give money to incompetent hams like me. The serial goes out later this year.

· Here's a joke which comes indirectly from Barry Cryer. All the good jokes do. I suspect he operates like De Beers with diamonds, releasing limited supplies on to the market at the most advantageous time.

Chap is in a luxury hotel suite in Manchester. He's had a line of coke, he's feeling frisky, so he leafs through a contact magazine and chooses a promising looking agency. "What I want is this," he explains. "I want a gorgeous blonde with a knock-out figure and a really willowy black girl. First I'll make love to them in turn. Then they can get it on with each other, and we'll round off the evening with a threesome. Can you do that?"

"Oh, Mr Deayton," says the operator, "if you need an outside line, you'll have to dial '9'."

· I didn't know Roy Jenkins well, though I met him often enough, and relished his baroque choice of words. He could use "otiose", "rodomontade" and the French "pièce justificative" in conversation, as easily as we would say "unnecessary", "boasting" or "excuse". I also claim to have defined the most famous of all his hand gestures as "a feudal lord cupping the breast of a plump young peasant girl", which brought almost as many angry letters to the Observer as the Guardian received this week for the Gillian Wearing G2 cover.

There's been a lot of lazy thinking about his role as a social reformer. Jenkins' liberal spell as home secretary did not lead to all this gun crime, and it's silly to suggest that it did. Nor did his crucial part as a backbencher in changing the law on obscenity in literature. It was he who got the Obscene Publications Act into law as a private member's bill. This was what persuaded Penguin Books in 1960 to publish Lady Chatterley's Lover.

In English law it is forbidden to have legislators explain in court what they were trying to do. Instead the court has to consider only the words which appear in the statute. Penguin's counsel decided to give it a go. Jenkins appeared in the witness box for the defence. His evidence lasted a few seconds. He was asked if it was his act under which the publishers were being charged, and he replied: "Yes, and if I had thought for one moment that this book..."

The prosecution's furious objection was upheld. But the point had been made. No wonder the jury cleared the book and its publisher.

· I hope to devote next week's column to your Christmas circular letters. Please keep them coming. A reader in Ilford got a letter from evangelical Christians which goes into far too much obstetrical detail about their difficulties in conceiving a child. But, they conclude, "If you want to walk on water, you have to get out of the boat!" Aaaargh.