It isn't quite like that for me. The story in my head is far ahead than my ability to write it. By the time I reach that point, I have everything in place for the story or the original idea was discarded.
However, it happens to me when I'm stuck. When I don't know what to do next, I go to the roof to breathe fresh air or the ideas come when I take a shower. I also reread part of my story or the whole thing.
When everything is going well and I have a new idea, it's mostly because of the things I have been reading. If there's something I disliked in one story, I think of a way of doing it better and a new idea is born. Most of the time is a whole new story, but there's a few occasions in which the idea is useful for my current stories.