It’s safe to say that your minor characters don’t know that they’re not all that important to your work. That doesn’t matter. I believe that if you want a well-rounded story you need to treat them as such.
Get to know them the way you know your main characters. What are their likes or dislikes, and most importantly, what is their motivation in the story?
Every person, every character, has a motivation. It’s what gives us purpose. It could be a rather mundane purpose, but we all have one. So do they.
They may be a minor character, but they factor in your story for some reason. Why is that? Do they help the plot? Hinder it? Do they drive your main character crazy? If so, why, and how does this shake up the narrative?
If you treat each character like they are the most important person in the work you’ll find a depth that you might have missed. Or, as I’ve found, you’ll realize there is a tantalizing sub-plot to be brought out.
I find minor characters just as interesting as major characters. I know why I’m writing about the major characters, but the minor characters always seem to have a kernel of possibility hidden away that is just waiting to be tapped.
I have two subplots in my current novel just because I looked at one of my major characters and wondered what he did after work.
In my opinion, no detail is too small, and no character too minor to have some attention paid to them.