Good question. I have been thinking about writing a book about my grandfather. He fought in France during WW1. Back then he was a young man. I know he worked on a Queensland station for a while as a Jackaroo. He was born in England and came out to Australia as a fit lad hoping to improve his fortunes. When war broke out he joined the Australian light horse. He was badly wounded and sent to England where he was expected to die. Instead he recovered, married a bar maid and came back to Australia. There were three boys and one girl. My mother was the girl.
My problem? I only knew my grandfather as an old man who made toys out of wood. He was a carpenter by trade. I only have what has been passed down to me to build him up as a young man new to Australia and then new in an Australian uniform. I have a LOT of research to do but I will have to fill in a great many gaps based on supposition. I cannot ask him pertinent questions. He passed away when I was ten.
I will not have strict proof of a lot of things to do with his life but I should be able to put together a fitting enough profile based on what family can tell me plus diaries, etc.
I have come across authors who have come clean in introductions about how little they know in some areas and how they have gone about bridging such gaps. I think this is fair enough. Also it is fascinating in itself to read.