In Bob Shaw's book on science fiction writing, he said that when he has told the gist of a story to someone, it releases some of the need to tell the story, and

In Bob Shaw's book on science fiction writing, he said that when he has told the gist of a story to someone, it releases some of the need to tell the story, and so it's less likely to get written. Therefore, when he's bouncing ideas off someone, and he gets a really good idea, he won't say anything about it, in order to conserve its energy, so it can be channeled into the actual writing.