Once again I'm digging into Karen Wiesner's book on the fiction series. Seems there are 3 ways series begin.
1. The Unintentional Series.- This has happened to me and probably to other writers of series. The stand alone book suddenly becomes a series. Why? First the character may so involve the writer that there are other stories for the character to appear in. Or the writer may find a world that totally interests them and they want to explore other parts of that world. Then, the first story can be so successful with fans the writer decides to write a sequel and another and another.
2. The Planned Series. There are writers who can sit down and plan a series from the first moment they begin work on a story. They may or may not use the same characters as the leads, but the leads are connected through some agency. Writing this kind of series takes planning and problems can arrive. While the writer may not know what each story in the series will entail they need to have a plan for the series from the beginning to the end.
3. The Publisher's Idea. This is a series that is a bit different. The publisher or an editor has an idea for a series and seeks the writers to do each of the books in the series. This can be hard or easy. The location is usually one place, a town a group of connected characters. While each of the writers does their own story, they need to fit their story to the overall plan. Some writers find this easy but others can't do this kind of a story.