Think about it. By the time you start writing your second book in a series you know your reappearing characters very well and so do your readers. That’s why it’s important to be consistent with your characters appearances, habits, body language and idiosyncrasies, not to mention what has happened in their lives previously.
It’s a lot to remember, isn’t there? The colour of their eyes, hair, settings they inhabit, people they love, grieve for, dislike. The list goes on. When you think about it there’s so much that I, at least, need help. That’s why I keep a detailed record of each of my characters. Two records, in fact. One for my returning characters which I update from time to time, depending on what is happening in their lives, and another for characters who have appeared in a particular book in the series. (You never know when you might want to bring one back!)
There are many ways you can do this. Having an index card system, keeping their details in a journal or, as I have done, creating a table in Word where character details can be listed in their own particular columns. It’s a quick reference point and easy to add things to.
What are your methods for keeping track of your characters?