Think of your brain as a toolbox. Think of the toolbox's compartments as tiny storage areas where you can keep what you need at hand. In one section, keep possible characters and their names. In another section, keep settings. In another section, keep likely conflicts. Keep voices in another compartment. Mix and match.
One day, pull out the unemployed factory worker, place him in an emergency room, and have a guy seated next to him holding his severed big toe in his hand. Another day, pick out a female stockbroker, place her in the stands at a rodeo, and have a little kid next to her accidentally jab her with the spurs he's wearing.
Carry your toolbox with you at all times.