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 ZelaznyGood evening, Lord Corwin,' said the lean, cadaverous figure who rested against a storage rack, smoking his pipe, grinning around it. Good evening, Roger. How are things in the nether world?' A rat, a bat, a spider. Nothing much else astir. Peaceful.' You enjoy this duty?' He nodded. I am writing a philosophical romance shot through with elements of horror and morbidity. I work on those parts down here.
Roger Zelazny