If you have a clear sense of how your story will end then you can, as it were, rewind to the beginning and plot any number of various routes that will allow you to arrive at that desired ending – with its attendant catharsis, of course. If you start writing (however striking your original idea) with no sense of how your story will end, then life becomes progressively harder. Flailing around. Writer’s block. Draft after draft. This is how novels get abandoned; film scripts bottom-drawered. The thing to do is to stop and envisage your final pages, your final scene. Take your time. What note do you want to strike? What surprise do you want to spring? What denouement will justify this journey?’
William Boyd, in The Guardian