Minor characters need attention just as much, if not more, than the main characters of your story. Why? Because they are even tougher to get right. If one inference to them doesn’t match up with another for no reason, then they aren’t a well thought out character. So how can you spruce up these minor characters without wasting your time on them? There are a few things to think about.
The first thing to keep in mind is the basics: what’s their name, what do they roughly look like? Are there distinguishing features that need to be mentioned when they begin to play a role? A great example of this is in Odd Apocalypse, Kenneth Randolph Fitzgerald Mountbatten is a minor character that turns up perhaps twice, maybe three times total in the novel and is not really mentioned otherwise, but he is a distinguished character with very certain and entertaining details of his character (including the long name) that flesh him out enough that you don’t question his being there. His physical presence and his speech pattern are well defined and talked about whenever he turns up in the scenes with the main character, Odd Thomas.
The second thing to think about is mannerisms. If they have a certain tic, nervous habit or tell about them and it comes into play, have it play a really big part of their character. Especially if someone else is focused on them (your main character, for instance), pick out a trait that will be most noticeable to whomever is your point of view at the moment. Stress this trait. Refer to it whenever you refer to the character.
Don’t forget to give them both successes and failures. If a character is an absolutely stunning masterpiece to look at, maybe they have an abrasive personality. This and the reverse are often used, no matter how active or inactive a character is. Go for balance, make sure they have flaws but they should also have talents as well. Don’t go overboard: again, try to choose one of each to focus on during their scenes (and that’s if they come into play at all!).
Consistency is going to be your biggest battle here. With minor characters it is difficult to give the audience a big enough impression of the character without wasting your time in fleshing out the character in all the same ways a main character would be. Try to make sure you are crossing your T’s and dotting your I’s, keep everything matching with that character. It will make for an easier, more immersive read for the audience.