In my brief experience so far, it's a mixture for me too. Sometimes I just start with a character or two and a story-type (eg spank shop) and go with it, while others have more of an outline and the spanking scene's been defined. The ending usually comes to me half way through.
Most fun for me is when it's like flopsy's experience and characters develop minds of their own; the story moments I enjoy the most all come spontaneously and surprisingly.

Even the much longer serial I've been working on grows itself and pulls me along, which often means going back and changing earlier bits so they fit the new dispensation. Alice Walker says at the end of TCP: "I thank all the characters for coming."