I found this good article with advice for creating characters by Lori L. Lake, and there’s a great section about creating secondary/minor characters (in roleplay we normally call these NPCs because nobody specifically controls them).
Most often, you need secondary or even minor characters to serve a purpose: they deliver news, convey information about the protagonist or plot, and end up as a sort of cardboard backdrop for the main action. The one-time, walk-on patrolman who reports to your detective hero might be stereotypical, but is there some detail you can ascribe to him to make him more interesting? Maybe he has a yellow mustard stain on his blue collar or he’s much shorter than a typical cop. Maybe he’s limping and that gives your protagonist a chance to comment on an injury she is bothered by. Or, say your main character buys flowers for her girlfriend every weekend. Perhaps the flower seller also traffics in Cuban cigars on the side and winks about it.
There is a limit to how much you can develop minor and secondary characters, but a few well-placed observations about them and their surroundings can do wonders to fill out your story and make both major and minor characters more compelling and believable.
Lori L. Lake (read the rest of the article here)