From image to story: how character grew a novel

My crime novel, The Fingerprint Thief, first announced itself as an image. For months it lived in my imagination as a blurry snapshot of a single moment – a woman called Sarah huddled over a body on a beach taking fingerprints with a shadowy, threatening city as a backdrop. It was like a black and white image taken by a toy camera that leaked light—dark edged, blurred, grainy. To make this image into a novel, to expand it from a suspended moment into a story, I switched from the poetics of imagination to the principles of narratology. I asked myself a number of interrelated questions. What is more central to a narrative: action or character? Do I start with the criminal events that brought Sarah to the beach or do I start with the story of Sarah herself? Once I have decided this, how do I make Sarah believable?