I've just written a 3000 word story based on a setting and concept that I've had floating round my head for several years. I was thinking about the plot on holiday last week and had the opening scene roughly formed, but couldn't decide which way to go with it. As I was sitting there my eyes lighted on a telescope, and suddenly the opening scene coalesced and I imediately wrote the first 1600 words.
Then I was stuck again. The plot arrived at a certain point, but I had no firm resolution in mind. The next scene came slowly to mind and I started typing it today. The two main characters fell into a conversation, and I realised gradually where they were going and what would happen. I finished at a completely unexpected destination, becasue that's where the characters wanted to go. It's only 3000 words but I really connected with theor world and their lives. It's a strange experience to have your story taken over by your own characters.