Example: I watch a lot of true crime shows. I watched one recently that was telling the story of this poor woman who'd been kidnapped by a man and his wife and imprisoned for six or seven years. I won't go into all the horrible details, but suffice to say that it sparked something. What I ended up writing had little, if nothing, to do with the original story - but it started the ball rolling. Another time, I was writing about a relationship between two characters, and I remembered when a woman in my life had asked me to sing her to sleep because she loved the sound of my voice. My voice is nothing special, in my opinion, but her love of it was so honest and genuine, I couldn't refuse. This found it's way into the book, even though the character I ascribed it to was nothing like the real life woman I'd sung to.
I suppose people (self included) look for formulas for things. An 'if you do A and B, then C will occur'. Writing is no different. In the end, I guess there aren't any easy answers, or, if you really want to get down to it - there are too many. 'Write what you know' is true sometimes. 'Less is more' is true sometimes. But 'rules are made to be broken' can be true in the right moment as well. It's all about a feeling in the gut, for me. 'This goes there.' 'Why?' 'Because it does.'
A last, too-cutesy little mention. If you want to get all butterfly effect and zen-ish. I once sat down and wrote out an idea for a story. It was a good idea, and, I thought, an original one. A few days later, I was watching one of those true crime shows, and there my idea was - in real life. Something that had happened years ago. I'd never heard the real life story, but the parallels were downright spooky. I'd come up with an idea for a story. A real life killer had executed this idea years before. Did we both get our inspiration from the same place?
Maybe, sometimes, ideas come from a dark little man in a dark little room who sings and claps his hands and laughs while we dance to his tune.