This was one of many questions she encountered in the five years of research and writing which produced The Secret River, out in the UK this month, her first novel since The Idea of Perfection which won the Orange Prize in 2001. Given the issues the book covers - the early English settlers in Australia and their clashes with the Aboriginal people - the keeping up of trousers is one of the easier questions at stake.
The book began as research into the life of Grenville's great-great-great-grandfather, Solomon Wiseman, a Thames waterman who was caught stealing timber and sent as a convict to Australia in the early years of the 19th century. She was researching it when she came to London to receive the Orange Prize in 2001, though at that time she thought the book would be a biography. However, she quickly realised there was a bigger story to be explored, which would need all her imaginative powers as a novelist.
"We in Australia have been very blind about a lot of our history," she says. "The things for which we can pat ourselves on the back, we do that a great deal, such as our performance as soldiers in various wars. But there is a dark underbelly to Australian-European history and that is our relationship with the Aboriginal people. I went into this book with fear and trembling, but almost for that reason, the fact that it went to a dark place, it grabbed me by the throat and wouldn't let me go.
"I felt an urgent need to write this book. When you're suddenly aware that your whole culture has lived with a secret place that it hasn't investigated, it's like a Pandora's box. The urge to open it, no matter what the consequences, becomes very great."
She knew she was stepping into the midst of her country's burning debate about land ownership, and into a battle which divides Australian historians into "black armbanders" (who acknowledge and seek to redress the mistreatment of native peoples) and "white blindfolds" (who claim the atrocities of the past were exaggerated and should not be raked over).
Grenville took her name out of the phone book for fear of repercussions, but the reaction to the book has been almost universally positive. Critics described it as history "newly minted" and said that it has helped change the way Australians look at their past. Grenville says: "I think we're growing up fast, and we actually want to hear these stories."
Though the book is dedicated to "the Aboriginal people of Australia, past, present and future", Grenville says she was determined to hold no agenda. "I didn't want to take sides. I didn't want to be accused of being either 'black armband' or 'white blindfold'. What I wanted to do was to step outside of that and say, 'Let's just imagine what it was actually like. Instead of judging these people, let's empathise with them, think what we would have done.' It's much too complicated to say they were either good or bad. They were neither. They were people just like us, fumbling their way through an impossible situation. It isn't a polemic, it's an attempt to understand a set of human beings.
"I got incredibly interested in what the life was actually like on a day-to-day level, to try to write my way into this almost unimaginable past which my family had been through. So I wanted to make it very much a book about the way the past smelled and tasted, what you heard, what you saw. I wanted it to read like a novel set in the present, not some historical novel with a carefully researched feel, to have the flamboyance and the kind of reckless drive of a good novel which just happened to be set 200 years ago."
Tracing the footsteps of her ancestor - the basis for the fictional character of William Thornhill - was easy at first. The transcript of his trial had survived two centuries, leading to information about his apprenticeship, and where his family had lived. "The wonderful thing if you have an ancestor who was a criminal is that they leave a paper trail. And although London has changed a great deal there are still corners where you can get a whiff of what it was like. If he had been an innocent man, not only would I now live in London, but I wouldn't now be able to find out anything much about him."
Once her great-great-great grandfather arrived in Sydney, however, it was a different story. The paper trail came to an abrupt end, and imagination had to take over. At first William and his sparky wife Sal see their new country as little better than the death sentence they narrowly escaped back home, "a place, like death, from which men did not return". But William becomes enchanted by the possibility of owning his own 100 acres in the unbroken territory of the Hawkesbury River.
Grenville had visited the past before, in her first novel, Lilian's Story, first published in 1985, which won the Australian/Vogel Literary Award, and in its companion book, Dark Places. But writing about well-heeled turn-of-the-century Australian society was a great leap of the imagination away from the on-the-edge struggle of the first settlers.
The writer pored over diaries and letters from the time, and over piles of historical books, but all that was no substitute for what she calls "experiential research": camping out on the still wild shores of the Hawkesbury, making a "slush lamp" from some leftover lamb fat, and wondering about ordinary everyday things like how trousers stayed up.
"The bush at night is a spooky place, and it made me understand how desperately frightening it would have been to come from the slums of London to this strange, creaking, whispering place. The physical difficulties were immense. I was nearly eaten alive. How did they manage before mosquito repellents and nets? That gave me a real sense of what it might have been like to be on the absolute frontier of Australia in 1810."
William Thornhill settles his land believing it empty, but soon finds it is viewed as a valuable source of food by the local Aboriginal population. He and his neighbours on the frontier must find a way to deal with these strange, naked, coal-black people whom they regard as savages, yet who seem at one with the land in a way which they could never be. One settler shoots them like animals, another integrates, intermarries. Thornhill must make his own choice.
Grenville wanted to know how her ancestor dealt with this dilemma, fearing "skeletons in the family cupboard". In the end, she could not find any written records, and had to imagine the outcome. Her account is compassionate to both parties, conveying vividly the life-or-death struggle faced by William, Sal and their growing family, barely equipped to make the life-changing decisions that confront them.
Although she wove the plot together, she says almost nothing is made up. "I did read a great deal of the local newspaper at the time, and most of the events I talk about did happen in one version or another. I felt like a real magpie, going through all this historical research and grabbing shamelessly whatever bits I wanted. It was a great freedom and a great responsibility to do that."
She discovered, too, that research can be addictive, and that there is a time to put the notes aside. "I would see so many things I felt should go in, and suddenly the poor book would lumber along like an overloaded mule with all these facts. The more difficult a fact is to find, the harder it is not to use it."
But she learned a great deal, including how trousers stayed up 200 years ago. "All the different mechanisms. So if ever zips and elastic disappear, I'm the person to come to."
The Secret River by Kate Grenville is published by Canongate, priced 12.99.