How do authors write book series?

I’m often intrigued by the various ways in which people write their book series. Do they write a well received title which they then see has an opportunity to further and make more money? Do they put words to print with sequels always in mind IF a book does well? Or, is a series of books always predefined, with individual novels simply being milestones between a predestined beginning and ending?

Such distinguishing isn’t always needed for all novel series. For example, many crime books are open and shut titles but have one continuing character who solves case after case as the series goes on. However, many series complete an overall story-arc, and it is this that intrigues me.¬† J.K Rowling always had seven books in mind, whilst the Mars Trilogy by Kim Stanley-Robinson were also predetermined. But do others have a set out quota of books before they sit down?

I’ve always found that my own writing process has been rather organic. Both of my WIPs are lengthy, and my start and end goals have always been pre-written into a number of books. I don’t know exactly, to the full-stop, how they will end, but I know the overall tone, setting and character developments which the ¬†books will close with. Other than that, I write each book with milestones to reach and specific moments to include, but a lot is left down to the characters themselves. In Inside Evil, for example, there are several instances where I was shocked at the route which a character took.

However, this organic process does allow for books to lose their way and become rather rambling, so a certain degree of structure needs to be taken; the reigns need to be held. In terms of developing a book sequence, it can soon become apparent in the book writing process, that the drama unfolding will never fit into one epic novel, and that the story needs to be broken down into smaller chunks.

I’m interested to see if other writers find that their book series evolve this way, or whether they write a sole title, note it doing extremely well and decide to pick up their characters where they left off. What works for you? Do your series create themselves, do you sit down with a clear number of novels to write, or have you simply attempted to maximise sales by taking favourite characters and giving them a new situation to play in?