17/02/2012 18:12 GMT | Updated 18/04/2012 06:12 BST

Why the Facts Really Count in Fiction

One of my stock lines, to be deployed whenever someone asked me about the difference between writing journalism and writing fiction, used to be: "The biggest difference is that in fiction you can't make it all up."

I wouldn't risk a joke like that now, not in the era of the Leveson inquiry into journalistic ethics and practice. Still, it did contain one element of truth. No, not an admission that as a reporter writing under the name Jonathan Freedland I've ever made things up - but rather a confession that, as a novelist writing under the name Sam Bourne, I've had to stick to the facts much more rigorously than I'd ever have expected.

I found that out early. Readers who could accept the wildest flights of fancy in a novel - heroes racing against time to avert international disaster and the like - would howl with fury if a character, say, boarded a Northern Line tube train at Green Park or fondly recalled the general election of 1998. Factual errors like that would have them hurling the book across the room, bitterly denouncing it as a pile of rubbish. For a small slip can bump the reader straight out of the novel and back into the real world. The magic spell is broken.

Which is why research is as important for my Sam Bourne alter ego as it is in my Jonathan Freedland day job. That was especially true for my latest novel, Pantheon, set in 1940 - in both Oxford and Yale - and firmly rooted in the historical record. To develop the story, I had to spend time in both places, consulting the archives, interviewing historians of the period - checking every detail as thoroughly as I would if I were writing my weekly political column in the Guardian.

Above all, I knew it was crucial to get the feel of 1940 right. So I waded through the invaluable Mass Observation study, accessible at the British Library, where you can find the diaries of ordinary people written at exactly that time. I wanted to know what those residents of Oxford who had not gone off to war would have eaten, how they would have got around, what they would have heard on the radio in the summer of 1940, when Britain was fighting for its life and genuinely feared Nazi invasion. Rather than guess at one character's phone number, I contacted the BT archivist, asking him to go through old telephone books for a certain village in that year. I know there will be mistakes - my father, a child in 1940, caught one or two just in time - but I hope I've kept them to a minimum.

Still, it is always a reporter's eye you have to use. When I combed through those Mass Observation diaries, it was the telling or vivid detail I was looking for - as every journalist is conditioned to do. When I spoke to those scholars in Yale, it was the arresting new fact Sam Bourne was listening out for, just as Jonathan Freedland would be when chatting up a politician.

And yet one part of Bourne's work is utterly different. With some ignoble exceptions, most journalists regard the quote as sacred: you can't put words into people's mouths. Yet novelists have to do that all the time. What a reporter would call making up quotes, a novelist calls writing dialogue. If I'm now getting the hang of it, that's partly thanks to journalism too.

That's because I was trained at the BBC; I spent the early of my career as a radio reporter for Today and the World at One on Radio 4. And there's one thing you learn in that job, constantly interviewing people, listening out for the 10 to 15 second clip of speech that will work on the air. You learn how people speak. To this day, I like to think I can always spot a fake quote in a print article - usually the phrasing is too neat, too perfect. I often draw on the memory of that when Sam Bourne sits at the keyboard, imagining a conversation between people who don't exist.

In other words, the working methods of Sam Bourne and Jonathan Freedland are not so very different. In fact, the two have a lot in common. Which is handy, since they have to spend an awful lot of time together.

Pantheonby Sam Bourne, the pseudonym of Jonathan Freedland, is published by Harper Collins, £12.99