The first book is finished and up on Amazon and work on the second well underway, developing an idea that actually predated the idea for the first book. I have a story line which leads on from the first book, explaining how a character that appears later in the second book got there.
But I can’t decide whether to use it or not. The character is not central to the second book, and his story doesn’t directly connect to its theme.
The conclusion I have come to is to write the story line anyway. It is an interesting story, introducing an intriguing alien character, on the basis that I might be able to use it somewhere. If I have it, I can use it, but don’t have to. If I haven’t written it, I have no choice but to not use it.
I suppose that this is the essence of writing. Maybe only half of what you write will end up in the final proof of a book, but until you have finished writing how can you know which half? It reminds me of the advertising maxim “Half my advertising spend is wasted; the trouble is, I don’t know which half.” Maybe only writing what I know will definitely be in the final book will limit the development of characters, and leave me with a poorer choice of material.
So, I will bang out the story, see where it goes, then see if it fits, or indeed is any good. Who knows, it might end up in a standalone novel!