I think every writer has a different way of handling backstories for their characters. In the beginning, I kept those data sheets many writing guides recommend using. They are good for you to list basic details of each individual, but it’s easy to fill up the page as more information comes out in the story.
As the novel progressed, that wasn’t enough. Several of my characters had long histories in my head and I wanted to write them down so no important details were lost. This was also important because the plots for future novels were developing in my mind and I needed to set them up. Basically, little hints were dropped in Darkness Haunts giving an idea for what might come to pass (or already had) for certain characters. It’s not meant to be a complete picture, but enough to get the readers wanting to know more. Further details of character backgrounds will be revealed over the course of the series and sometimes their past will return to haunt them. What series doesn’t have this happen? It won’t be too cliché, though. I promise to make it fun.
At this time, I have the plot written for book two, a solid idea of book three, and a vague idea for book four. It’s entirely possible a fifth book could be conceptualized, but I’m waiting to see how things go. There is already a lot of material to work with, so I’m sure ideas won’t run out too soon. It’s not uncommon for many writers to not know exactly how far they plan to go before stopping. I do like staying in one world so I can continue to build my characters and add new dimensions to them. That the story doesn’t have to be confined to one book, though I may try and write a stand-alone someday.