So you know that you need to fully develop a background and motivation for both your antagonist and protagonist and of course, their supporting cast (sidekicks, best friends, and close confidants). But how much do you need to develop minor characters?
Well, that all depends on how minor they are. If they are just part of the setting – say the surly teenager at the coffee shop who tosses the sticky bun on the tray and nearly spills the coffee, then I would say pretty much no development is needed. This character almost doesn’t even warrant a name – unless that name is going to play some role in the story such as reminding your character of their first boyfriend which could lead to some back story.
Now if this minor character gets a speaking part (beyond “excuse me” or “How can I help you?”) then it might be helpful to have a few facts about him or her. In Destiny, I introduced a character that only appears in two scenes, though she is mentioned a few other times. She is a young thief who has great drawing ability. Well of course she needed a name (Mira) and a description (a shy, thin girl of sixteen with long brown hair) as well as a few facts such as she is a better artist then a thief. But I didn’t create a complete back story as to why she is living on the streets or working for the thieves network when she isn’t a good thief. I know nothing about her family or her life before she enters my story. And it works as she is a such a minor character. There is no need to waste your time developing a character profile on such a character but you might consider giving your minor character a distinctive trait. In Mira’s case it is her shyness that stands out.
Now in Quietus and Destiny I have a lot of characters that are minor. There are High Council members (basically government officials) and other Elementals (people who can control the elements) that train with my main character Lina. In this case, since they appear numerous times and directly impact the story, I did develop brief character sketches so that they would come to life in the story. Especially with the Elementals, I needed to know what element they could control and where they came from (not just country but employment and family) as well as what they looked like and how they behaved. Of course for most of these characters I only devote a few lines to each. But knowing these facts I think makes it easier to write about them as if they are real people rather than just people to advance your story.
So how much time you spend on these minor characters depends on their role in your story. In some cases knowing their motivations (why they are helping or hindering your protagonist) can prove to be very beneficial. In other cases, you need nothing more than a description.