Have you ever read a book where something about a character's appearance changed mid-story with no explanation? The character is first described as having blue eyes and then they're later described as brown? The hair color changes or in some cases, even the person's name suddenly becomes something completely different? Writers work hard to catch these type of things but occasionally they slip through the editing process even at the final publication end by major publishers.
It's even harder to keep characters and their appearances straight when you're writing a series with multiple characters. Main characters in one story usually become secondary characters in other stories in the series. And then there are the secondary characters who will always stay that way but are needed to move the series forward.
I'm currently working on my first series. The first and second books are written. They still need to be revised before being sent out on submission though. I didn't have a problem remembering someone's name or what they looked like because I went right from finishing book one into writing book two. I took a few weeks off from writing to let those sit a bit and loosely plotted out the story lines for the rest of the series.
Deciding I would write the third book during the week and revise book one on the weekends, I jumped back into the writing fray. And suddenly had problems remembering the appearances of some of the characters from book one, some of their mannerisms, and the like.
I've read multiple times in multiple places in the past that one should always keep a "Bible" when working on a series and include character sketches, character backgrounds, setting details, world building details, etc. in a format that's all neatly indexed and tabbed. Did I do that? No. I had to wade through the first manuscript looking for the details I couldn't remember.
My writing time is at a premium and using that precious time to build a notebook full of information I wasn't sure I'd even need didn't seem important. I also didn't see how I could forget anything about my characters or where the stories were taking place. I quickly learned how wrong I was.
Boy, is that series bible important. I still haven't put a physical notebook together with all the pertinent information I think I'll need, but I am building a spreadsheet in Excel. My spreadsheet includes character names, hair and eye color, distinguishing things about them such as tattoos, scars, accents, their family members, and a bit about their background that makes them they way they are. Setting details such as street names, parks, coffee houses and the time of year are also included. I've started keeping research details on a separate tab in the spreadsheet as well.
I learned an important lesson. All those writers who stated that a series bible is an absolute necessity knew exactly what they were talking about. I can't help wondering how many of them learned this the same way I did, by trial and error so to speak.