Take what you see of them already and form a character sketch. Stick people who resemble people from your past in. Take somebody whom you hated a long time ago and try to write them in a light that you would actually want to associate with them. If you are so inclined, keep a file of character sketches in your computer and pull them out when you need one. The characters must change through the course of the story.
Characters are critical to the story.
If they are nothing but plot operatives, you end up loosing realism. You need to consider why a character reacts the way he does in a story, because this character is not you. For minor characters, all you need is an archetype. Minor characters generally don't develop or grow, but are only a tiny step higher than the scenery. If your situation and the character's reaction don't work for your story, you have two choices.