First, think about the specific details and descriptions for characters that make them come alive. Write them down.
These might be things like dialog quirks (I'm thinking of how Gatsby called everyone, "Old boy," or maybe an accent), repeated gestures or habits (carefully removing onions from any dish, drinking white Russians, tapping his or her fingers on her chin), unique appearance, specific smell, history and background, and family.
Add more. What are the details about characters that are most telling about who they are?
Second. This will take a little creativity and flexibility. Take the list of character traits and apply them to a place--a specific place will work better, like a city or a town or a neighborhood--perhaps even a house or a restaurant. Maybe a dive bar.
What dialectical quirks does the setting have? What gestures and habits do the people have and why have they developed these patterns? What does it look like and smell like and why? What is the history and the background? How did the people that live or go there come there?
Of course, writing a good setting won't necessarily come from a writing prompt, but here goes. Randy, of course, has already discussed this, and he knows his settings like he knows his characters. But like characters, settings can benefit with a good character sketch.
Know your places. Keep going.