As far as randos, I'll kinda just experiment and see what kinds of features I can actually draw on a certain day.
(You can kinda tell here which characters are gonna reappear according to detail/differences, despite the awkward style phase.)
But in case I like drawing that character, or they appear more than a few times (due to environments or MC situations) then they become more recurring as I go along the plot.
Then I'll come up with brief info on their role, when they appear, and what they contribute to the plot/mains. Nothing too detailed, yet.
Example, Dr Fowler was just an imitation of a plague doctor, and was meant to appear once in the entire series, but as I developed future plot points, I felt his inclusion would serve a greater purpose, so he's gonna reappear and become more active at a certain point.
Not to mention, my style has improved a lot, so he deserves to be redrawn!