A few years ago, I was staying at a friends house. At the time I worked third shift so I was awake much later than everyone else. I sat in one room reading for several hours. The house was silent except for the sound of a television in another room. It was quiet enough that all I could hear was the occasion sound effect and bits of muffled dialog. I kept wondering if my friends were ever going turn the television off. Eventually I wandered out of the room I was in, and when I did I stopped hearing the TV. I assumed that someone had woken up long enough to turn it off, but when I went back to reading, the television sounds returned. Eventually I realized that I wasn't hearing a television, I was hearing the gurgling of a table top fountain. It was the aural equivalent of the optical illusion of the vase/two faces. When I listened one way I could hear voices, when I listened another it was nothing but water gurgling.
Once I realized the nature of what I was hearing I discovered that I could hear what I was expecting to hear... I could find words that I was looking for. I could hear my own name in the gurgles. Honestly, it was really creepy. A few weeks earlier, I had read about the 'electro-voice phenomena'. Simply put, people who believe in the EVP believe they can hear the voices of spirits in electronic static. When I heard the voices in the fountains I came to the conclusion that I knew how the EVP worked... the brain is very good at finding patterns in things, especially patterns it is expecting to find. Sometimes it can even find patterns when they aren't there.
I plotted a story about a charlatan who set up a 'whisper garden' where people could go to meditate and commune with the dead. In that story idea, the central character didn't believe the voices were spirits, but they eventually drove him mad. In that story, the whisper garden was going to be in a rural area, and located by a natural waterfall. I never got around to writing that story, but the idea kept tickeling the back of my brain.
Then I went to New Orleans, and had another story idea. This one was about a drunk who would take the offerings off Marie Leveaux's tomb.
Somehow the two stories came together into one, and a cast of characters entered my head. The whisper garden is part of the plot, but the main story issn't about the EVP. The story also contains a drunk who steals offerings from a voodoo priestess' tomb and sometimes acts in ways that answer the prayers of the people who had left the offerings, but the novel isn't about voodoo. On one level the novel is a thriller about a young couple's struggle with a brutal killer. On another level the novel is about the way that seemingly unrelated things relate.