Jane Marple, like Christie's other famous detective, Hercule Poirot, is an amateur sleuth. Both tend to be one step ahead of the police, and both have a way of getting witnesses to talk to them. But here's what Christie understood so well about her two characters: they were outsiders, and as outsiders occupied a unique position--that of observer.
Among the English upper crust, Poirot is a foreigner. His slicked-back hair and waxed mustache are a joke, as is his accent. Those around him--including the various murderers he foils--don't perceive him as a threat. He's not one of them, so they ignore him. They don't reckon on the fact that nothing escapes his notice.
I'm a Miss Marple fan, but I wasn't always. As a young reader of Christie, I had no interest in an elderly lady who sits in a corner knitting, and therein lies her power. Then, as now, elderly ladies are all but invisible in society; they usually hold little power, and they are easily dismissed by others (as is the case of the woman in the book I'm reading now). But they sure as hell pay attention, something I appreciate much more as I get older. Miss Marple, with little to do except watch people, has an understanding of human behavior beyond that of the various Scotland Yard inspectors she foils.
When I set out to create an amateur sleuth of my own, I made her a mystery writer. (In fact, Victoria's main character, Bernardo Vitali, might be considered the Italian version of Poirot.) As a writer, Vic is also an observer. She takes in the small details of physical appearance and personality that others might miss. And as a writer of mysteries, she's conversant with the why and how of murder. But unlike Poirot and Miss Marple, she makes her share of mistakes.
As does her creator. . .