One of my standard - and fairly true - responses to the question as to how story ideas come to me is that story ideas only come to me for short stories. With longer fiction, it is a character (or characters) coming to visit, and I am then obliged to collaborate with him/her/it/them in creating the story.
Roger ZelaznyI watched the spinning stars, grateful, sad and proud, as only a man who has outlived his destiny and realizes he might yet forge himself another, can be.
Roger Zelazny