A few years ago I woke up with an idea for a new story. I jotted a few notes down so I wouldn't forget the general idea and headed off to begin my day. Since I was already working on revisions for a book under contract, I would write things down about the new story whenever they popped into my head. When I finally was able to sit down and focus on this new idea, I realized I not only had a story line for the main characters but also vague story lines for at least 3 of the secondary characters.
I'd never written a series before so I plunged ahead and wrote the first story. Once that was done, I set it aside and wrote the second story. After that one was done, I wrote about the first 100 pages of the third story and then ran into a brick wall of uncertainty. Not the kind of uncertainty that whispers in your ear in the middle of the night that your work will never sell, but the kind where I couldn't remember the most basic details from the first story that needed to carry through the series.
My writing came to a screeching halt as self doubt crept in. How could I forget the eye color or hair color of the heroine or hero from the first book in the series? How could I forget the most basic premise the whole series was built upon? And more importantly, how the heck did writers who have multiple books in a series manage to keep all these details straight?
I set the manuscript of the 3rd book aside and started revising the first draft of the first book in the series. I had intended to do that after I finished the second book, but the words were flowing for the 3rd book and I didn't want to lose those ideas that were coming so quickly. Now as I'm editing that first book, I'm putting all the pertinent details about each character whether they will have their own book or not into a spreadsheet. These details include physical descriptions, character traits, occupations, background information about their childhoods, their parents, and the key event that has made them the way they are today. I've also included setting details and anything else I think I might need for a book further down the road.
At this point I don't know if I'll write more than one series but if I do, I'll know to keep track of such details at the beginning instead of later and save myself a lot of editing in the process.