The secret to writing a best-seller is ..

Frequently they have to tread a fine line creating believable adventures that will keep their readers glued to the page.

Take Quintin Jardine's latest novel Lethal Intent, for example. The preamble on the dust jacket reads: "Four ruthless Albanian gangsters have infiltrated Edinburgh's underworld and MI5 are all over it. They believe the criminals are trying to move into the city's drug scene. But do they have a bigger, more audacious objective?"

Instantly, the recent discovery of a crack-cocaine factory run by Colombians in a Leith flat springs to mind, but there the similarities end - in Jardine's Edinburgh, drugs are the least of Edinburgh copper Deputy Chief Constable Bob Skinner's problems.

So what is the is the secret of writing a believable thriller?

If anyone should know it's Jardine, his hero Skinner first appeared in Skinner's Rules in 1993, proving so popular with readers that he has since returned for a total of 14 adventures in just 12 years.

Jardine believes Skinner's strength lies as much in the development of the character over those adventures as it does in the plots - a process that continues on Monday when the DCC enjoys his 15th outing with the publication of Lethal Intent.

"In this book he has a failing marriage and a potential new relationship in his private life. Professionally he's got this sudden threat to cope with and alongside that there are the usual two or three other things going on.

"But the core of Lethal Intent is this Albanian thing. They've turned up in Scotland. What are they doing? The police think they're drug dealers, but Skinner's not sure and sets out to discover what they are really up to."

If it sounds as if the tale includes more than a pinch of good, old- fashioned soap opera, Jardine is unrepentant.

The writer, who will launch the latest Skinner novel at Ottakar's Bookshop in George Street on Monday, insists: "All long-running crime series have got to have an element of the soap in them. They are character-driven as much as plot-driven. And while I don't set out to write soaps, it's important to make your characters three dimensional.

"Skinner is a prime example. He had a brother who wasn't mentioned until book 12. His mother had a drink problem and his father was a war hero and Skinner's been trying to live up to him ever since.

"Then there's his wife's background and how things we know about her have changed over the years - all along Bob's been second best."

As with Jardine's previous novels, Lethal Intent began to take on a life of its own as the author started to put it down on the page.

"With Skinner, I switch him on and he takes over. He tells me what to do," he explains. "All sorts of things can happen during writing . . . plots change, characters walk on the page and invent themselves.

"Towards the end of Lethal Intent something just happened on the page and a character got killed. Somebody who has been in the series for quite a long time. Now I didn't start the book meaning to bump them off but it just happened. The story was there on the page and I just thought: 'What If?'

" 'What if?' is the greatest thing that can happen to a mystery writer. If you keep the 'what ifs' in your head, they can take you to all sorts of places."

It's an approach that has proved a hit with readers and just secured Jardine what he describes as "a nice chunky six-figure advance" for a new four-book deal.

"That means that, God willing, there will be a new Skinner in the shops every year until 2010."

If crime writers have to work within the parameters of a reality, albeit a heightened one, that their reader can relate to, the same cannot be said for science-fiction authors. Their only limitations would appear to be those they impose upon themselves.

For them fiction is definitely stranger than fact - though not always, cautions Iain M Banks, who writes both mainstream and science-fiction novels - the M disappears when he slips into contemporary mode.

"That's the trouble," he reveals. "As an author, what you write has to be plausible, whereas reality doesn't have the same constraint. It can be as stupid and ridiculous as it wants. It's full of bizarre and unlikely coincidences because that's just the way it is. That's life.

"One of the techniques of getting around that, of course, is to remind people how ridiculous reality can be, but you can only take that so far - it won't save you from having something really stupid going on in your plot ."

This week sees the publication of Banks' latest space opera The Algebraist,in paperback.

Set in 4034 AD, when humanity has made it to the stars. Fassin Taak, a Slow Seer at the Court of the Nasqueron Dwellers, has been seconded to a military-religious order he has barely heard of and sets off in search of a secret hidden for half a billion years.

It's an adventure of epic scale, exhilarating escapism and outrageous names.

"The kind of science-fiction I write is not near-future sci-fi, so that makes it quite easy for me. Writing far future science-fiction is more like wish fulfilment. I just think of all the gadgets and technology I'd like to have and hey presto, there it is," laughs the author who will be discussing and reading passages from his latest novel at the Pleasance Cabaret Bar tonight.

"In SF I can get away with more and just make things up, whereas in mainstream writing I'd have to do the research," he adds.

But having free reign does have it's drawbacks.

"It's a delicate line. What you are trying to do is keep the 'gee, gosh, wow' element without the reader going 'that's just stupid'. One way of doing that is by having a degree of consistency. Don't just make up laws and then trample all over them willy nilly because it happens to suit the plot. That's what gives cohesion to the whole thing."

Iain Banks, Pleasance Cabaret Bar, The Pleasance, 6.30pm, today, 1 (ticketed), 0131-622 8222

The Algebraist by Iain M Banks is on sale published by Orbit (paperback) price 7.99

Quintin Jardine, Ottakar's, George Street, 7pm, Monday, 1 (ticketed), 0131-225 4495.

Lethal Intent by Quintin Jardine is published in hardback on Sunday by Headline, price 18.99