I didn't start with such big ambitions for the first series, but as I added characters and episodes and cases, new plot-lines simply begged to be included. And some of the characters demanded bigger roles too: Sharon Dunbar, Dougie and Giovanni were initially to have minor roles, but I liked their characters too much not to use again.
And things like the Colonel's history, Timothy's skill on computers and Clemence at the Art Gallery ... while these were all planned from the beginning, I hadn't anticipated how quickly they would develop into something an awful lot more than I initially intended. It was only by working through the threads that I was able to weave them into a patchwork of story lines that cross and clash across the series.
Of course, managing the patchwork could only be achieved on a spreadsheet. And it's a pretty big one too. I set it up with one column per episode and one row per story thread. Then I developed each thread across each episode to map out the entire series.
Detailing the characters was also essential. I have a profile on each of them that details their age and birthday, what they look like and any singular traits they have. If I'm writing and decide (for example) that Dougie smokes because it fits the scene, then I add that information to his profile.
Knowing the age and date of birth of each character also helps. I may not mention every character's birthday in the series, but I will use it if it's relevant to the time of year and activites I'm writing about.
The Bailey books are set in 2018 and each date is set on the actual day from that year. When we first meet Bailey it's Thursday 10th May. That's not important to the story, but me knowing it means I can slot in the weekend at the right points. And if there's a Bank Holiday or school holiday, I can use that too.
I have a separate spreadsheet covering the timeline of events. It dates back to 1999, before Bailey was born. In it I have key events logged such as her parent's activities and actions that led to the end of their marriage. This timeline also details every key action of Bailey's journey in the books: splitting up with Paul, arguing with her brother and meeting Matt, etc.
So, if ever I need to check a fact or character placement at a certain time, it should all be there for me in a spreadsheet.
At least, that's the theory.