Writing seven books over 20 years based on the same characters is no problem for Diana Gabaldon.
The best-selling American author was in Dunedin this week talking about An Echo in the Bone, her latest novel in the "Outlander" series, which follows the lives, loves and adventures of Jamie Fraser and the time-travelling Claire Randall Fraser in the 18th and 20th centuries in Scotland, the West Indies, France, England and America.
"Jamie and Claire are what I call onion characters, whose essence I apprehend immediately but who develop more layers the more I work with them.
They become more rounded and more pungent with time; like anyone who gets older, their life develops additional complexity," she says.
Most of the story takes place in "interesting times" of political upheaval, particularly through the 1745 Jacobite uprising and the battle of Culloden in Scotland, and the American Revolution in the 1770s.
By the end of An Echo in the Bone, the protagonists are in their late 50s - the same age Gabaldon herself is now.
"In my third book where Claire was 47 and she went back [to Jamie in the 18th century] people were writing to me and saying, `Do you think you can possibly have an interesting story about people this age?'.
I said I was 43, which I was at the time, and I was not planning to stop having sex any time soon, and neither was my husband if he knew what was good for him - and that still goes."
Because she does not like doing the same thing twice, she makes each book different in terms of tone, structure and approach, and the characters change and develop, but the books all share the same great attention to detail and meticulous research.
An Echo in the Bone has four story lines running through it.
The main one, of course, is that of Jamie and Claire - and their family, friends, acquaintances and enemies - and their adventures during the American revolution, she says.
Alongside is the story of their daughter Brianna and her husband Roger, who have to return to the 20th century to seek medical help for their younger child Mandy, who is born with a heart condition.
"They are settling into life as a young family and struggling with the questions of professional identity and personal relations, and at the same time they are beginning to be aware that the Highlands of the 20th century might not be any safer than the Highlands of the 18th as things creep in on them."
The third thread is that of Lord John Grey and his stepson William, who is Jamie Fraser's natural son, although William would be appalled if he found that out.
"Through them we see the British side of the American Revolution.
Many US readers would not be aware that war had two sides, but it did, and as a good historical novelist I thought I'd show you them both."
The fourth story line is that of Ian Murray, Jamie's nephew and foster son who was adopted into the Mohawk tribe of Native Americans, but returned to the white world, which brings its own conflicts, she says.
The device of time travel, through ancient standing stones at certain times of the year, means you have to work out the mechanics and logistics of how it works, and beyond that, the moral and philosophic consequences, she says.
"If you are a time traveller and you know what is going to happen, have you got an obligation to try and stop something bad happening, that you know about?" she says.
"If you do try to change the past, what happens to you as a consequence?
"That has a concomitant parallel streak, and the point here, and throughout the books, which is unspoken, is the same problems facing a time traveller are the same choices that face us all.
"You always have to decide, `What am I going to do', because what you do affects your future and that of people around you. That is always true.
"It's just that you have no particular knowledge aforethought but you still have to make those choices, and you have to make them on the same basis: usually, what is moral? What is right?
"Can I do wrong in order to prevent greater wrong? Must I speak out if I see something I think I can do something about? If I do, what will happen to me? What will happen to the people that I love?
"That choice is always with us. It's just that you can make it a little more pointed if you have time travel as a backdrop."
Gabaldon, a former academic and zoologist, started writing fiction in her mid-30s.
Although she had three children and was working two jobs so her husband could set up a new business, she decided that if she was ever to write a book, she had better get started.
She retired from academia in 1992 when the second was published.
It takes about three years to write each of the 800-1000 page books, not only because of the length, complexity and the detailed historic research needed, but also because she engineers them so those who have not read earlier books in the series will find enough back story to give a satisfactory reading experience, and those who have read them won't be put off because they know it all, she says.
Given the cliff-hangers at the end of An Echo in the Bone, there will certainly be another book in the series, but whether there will be a another after that, she says she doesn't know.
"I have no idea until usually 18 months to two years into writing one what the boundaries are going to be in terms of how much time and ground I can cover.
"Certainly there's one more after An Echo in the Bone, but whether there'll be an additional one or not - there might but I can't say.
"We do in fact have all these people whose lives are involved here, and have to address all these people."
However, she doesn't plan to write a multigenerational saga.
It's Claire and Jamie's story, and if they should die, that will be the end.
Although obviously she has to take care of all the other people, she says.
Gabaldon's books have always been difficult to categorise, something that probably puzzles marketers rather than readers.
In the early days, when she was signing books in malls, people would ask what kind of books they were.
"For a long time I would reply depending on who the person was. If it was a young woman in her 20s, I'd say, `It's a historical romance; you know, men in kilts'.
"If it was a young man, I'd say, `It's fantasy, time travel, some battles, Scottish swords, all that'.
"If it was a slightly older woman, I'd say, `It's historical fiction, if you love Shogun you'll like this', and if it was a slightly older man, I'd say, `Science fiction - I've been asked to write up the Gabaldon theory of time travel for The Journal of Transfigural Mathematics, in Berlin', which is in fact true. I did.
"And if it was an older man still, in his 60s, I'd say, `It's military history', which worked fine."
It certainly sold a lot of books, she said.
An Echo in the Bone by Diana Gabaldon is published by Orion (pbk, $40).
