SOMETHING different today. I am over in Knutsford to talk to my mother’s U3A book-reading group. So here is an edited version of what I’ll be saying…

My name is Julian and sometimes I kill people. Now I am quite a peaceable person really, so why do I do it? And how do I get away with my crimes?

Well I only kill people on the page, you see. I still feel guilty about it sometimes. You might have invented these characters but they are real to the reader. Or they should be. So killing them counts for something.

Once I killed an elderly German pilot who came to York to apologise for bombing the city during the Second World War, and I did feel pangs about that.

The reason for my guilt was simple enough. A German pilot did exactly that in real life – and I borrowed the incident for The Baedeker Murders. Only in my version he is murdered during his stay. But that’s what crime writers do – they take something ordinary and turn the setting to ‘dark’.

So, yes, I have killed a few people – although not enough according to one American critic, who said there were too few murders in my books. Well you can’t please everyone.

Crime is popular, as any visit to a bookshop will tell you. What a lot of crime books there in print. And these books fall into all sorts of categories, from what is known as ‘cosy’ crime to the more hard-bitten books.

What unites all these very different novels is that a crime has been committed, usually a murder. There are various appealing elements to a crime book, but part of the attraction lies in the puzzle. Sometimes this is as finely made as an old-fashioned watch, with all the cogs, wheels and springs busily working away.

A classic example of such a book is Agatha Christie’s And Then There Were None, in which a group of strangers receive a mysterious invitation to a remote island. They never meet their host, who has used flattery to draw them in. But once they are trapped, the unseen host reveals that each of his guests has committed a crime in the past for which they have escaped punishment. And one by one, the guests are murdered until no one is left. So who can the murderer be if they’re all dead?

That is a closed-room mystery, a seemingly impossible puzzle with a neat twist or conclusion. Add a remote location, such as an inaccessible island, and the setting is perfect. No one can leave and no one can arrive.

Sometimes modern crime stories use the old-fashioned approach. A very good writer such as Sophie Hannah weaves her psychological crime thrillers from the old Christie cloth.

And then there is Iceland. Recently on BBC4 there was an excellent Icelandic crime drama called Trapped. The title said it all. The characters were trapped by a snowstorm – and in other ways too, hemmed in by the cold claustrophobia of their lives.

This was, in a sense, an old-fashioned Agatha Christie story set in a strikingly different location. Agatha with ice and a gigantic cop who mumbled from behind his beard. Instead of a closed room or island, there was a small coastal town made inaccessible by the weather.

There will be blood in a crime novel. That goes without saying. How much is a matter of taste for the writer and the reader. Too much violence can be off-putting for many readers – and some writers, too.

The crime writer Mark Billingham says that the single spot of blood on the floor is much more frightening than splattering blood all over the place. That single spot soaks through to our imagination. Show too much and the mind shuts down; hint at what might have happened, and the reader’s imagination joins up the (bloody) dots and finishes what the writer has started.

Another popular writer who has often addressed this matter is Val McDermid. I’ve heard her speak many times and this usually comes up. Val addresses a seeming paradox of crime novels: these books are very popular with female readers – and yet the victims are so often women? So why do women like books that inflict hurt and injury on other women?

Val’s answer often takes two parts. One is that reading a crime book is a safe thrill, like riding a rollercoaster. And readers like being scared, we like the adrenalin buzz and the excitement of the floor dropping away beneath us. Her other answer is that women have more vivid imaginations than men, and that they like to scare themselves by living through their fears in fiction. They are better at imagining terrible things and draw an odd sort of comfort from approaching their fears through the page.

For the writer, the crime book can be almost anything you wish it to be. There are rules. If there is a crime, there should be a resolution of sorts. And you should not play unfair tricks on your readers. The solution should make sense in the terms of the book. You can’t end a story by saying than a previously unmentioned alien did it. That would be cheating – as well as ridiculous.

But within the broad framework, there is room to do almost anything. And to explore almost anything. The darker side of human psychology, for sure. Or perhaps politics and sex and the wrongs of the modern world.

Crime books will also be about morality to an extent. Immorality will be tracked down and hopefully punished. But not all endings have to be neat. Sometimes a lack of complete resolution might be satisfying in a disturbing way.

The question that most amuses or annoys writers is: “Where do you get your ideas from?” One of my favourite writers is Ian Rankin, the creator of Rebus. He once became so tired of being asked this question, that he told a man in the audience that there was an official website where accredited writers could go and download all their ideas. The irony was lost on this man. He came up afterwards and offered Ian £200 for the web address.

My ideas come from here and there, and from history too. My first book, The Amateur Historian, concerns a girl who is kidnapped in modern times – a crime linked to the death of another girl who lived in poverty 100 years before.

That was inspired by the Seebohm Rowntree report on poverty in York. I read the report and soaked up some of the facts and figures, and retold them as fiction.

When I write I have no plan as such. I have a beginning, a shape to follow, a rough map in my head. Usually I have an ending. And then I just set off to see what happens.

Other writers do this too – although many prefer to plot and plan. I’ve tried that but it doesn’t work for me. So I pick up an idea and go with it. Somehow the writing brings the story alive in a way that planning doesn’t for me. I just follow myself into the dark…