Well to be fair, characters and setting do contribute to the development of the plot.
For me, most of my ideas begin with a scene that I gradually build onto and flesh out, and the details of the characters, setting, and the basics of the plot grow organically from that.
I always have tons of ideas I just let simmer on the back burner awhile, revisiting them now and then to rework them a little until all the important bits click into place. I have to have a pretty solid idea of how things are going to go before it turns into a 'yes, I am going to write about this' kind of thing.