Your protagonist will be surrounded by a cast of supporting characters in various settings. A few of the supporting cast will be main characters, each one nearly as important to the plot as the protagonist himself. Other characters will have a smaller role to play. Still others will be walk-ons, put on the page to make the setting feel authentic. You'll need to think about creating a worthy cast and setting to support your plot and your protagonist. Can you imagine Gone with the Wind without Atlanta under siege during the Civil War? Or Dune without the desolate desert sands of the planet Arakis? Or Wuthering Heights without the moor? In novels like these, the setting (time, place, context) is like another main character.