I have thought about this while thinking about a game concept, but it should be applicable to many game types.
Imagine an RPG or something else with an open-world. Normally you would want to populate your world to make it feel alive and allow for interaction with NPCs in any form (talk, steal from, kill.. whatever).
Now, what if you don't have the option to fill it with NPCs or similar. My example would be a procedural generated RPG world where you want to enter a house filled with people or even a city. It would be strange for none of the NPCs to talk to you.
Another example might be a space station your ship is docking to. There might be only one or two NPCs you are able to talk to, but the space station could be steaming with other people, but you might not acces them somehow.
Sure, one possibility would be to have procedural generate NPCs with random messages as well, but depending on the size of the city and/or the amount of NPCs you would need a lot of NPCs with different messages. And then it would be easily possible to have to NPCs with identical messages. The other problem would be that you would end up with to many NPCs, that make the place feel overfilled instead of just crowded. I image that a hard number or relation for space to NPCs is impossible to pinpoint, but you surely get my point.
So, my question is, what other possibilities are there to make a city / place feel crowded? Maybe with NPCs, or maybe even without?