You're able to spawn an npc (whether it's a police, civilian, gangster, etc.) through the npc spawning menu. When you spawn one - it isn't like any ordinary NPCs but it also... acts like one of those NPCs being spawned, if that makes any total sense.
I get kind of sick having to manually spawn NPCs or rely on luck for a NPC to spawn in a specific place I want them to be. If I were to spawn a normal NPC they wouldn't have the same reaction and behaviour the Background NPCs would have when I kill. Etc, etc.