One of the fun, or frustrating depending on your viewpoint, challenges of starting a game in a modern or futuristic setting is populating your world. This is a unique challenge for these setting types because when you run in a fantasy world there simply isn't as much communication and thus less interaction between characters before a face to face meeting. In a modern setting though, well, think about it. You are regularly bombarded by broadcasts from people all over the place, and kept in touch with people your friends like through social media. It can be a challenge to capture the feel, but done right it can really help to sell a living, breathing setting.
Characters For Use
The first place you want to always start is with the characters you intend to use pro-actively in your game. Not only are these the characters that are going to get the most screen time in your game, but they're also likely going to set the foundations of interaction between your players and your NPCs. Also, since they're going to be used a lot you're often best coming up with them - and building the world and other NPCs around them - than you are doing it the opposite way. After all, if you fill a jar with sand first you'll never get the rocks in.
Everyone knows how to make these characters - for the most part at least - because we all use them. So, I'm going to move on. (Let me know in the comments if you'd like something more in depth here.)
Characters For Idol Interactions
These characters are a lot harder but you can also skimp on the work. These are the characters who may show up regularly but don't necessarily do anything super important for the plot of your game. A neighbor who checks in on the PC from time to time, an ex-girlfriend, a fellow adventurer who travels with a different group. They can be useful, but their interactions are limited and usually more in the calm/R&R kind of way than anything else.
For these characters you just want a few basic details. Know what they do and why they are in the area the PCs are most likely to see them regularly. This will help you identify why the players may want to interact with the character regularly, and more importantly how the players may intend to use the character. From there, give the character a personality but don't feel the need to go too in depth right from the beginning. After all, this character is intended for casual interactions and we don't often give too much about ourselves away in those interactons. So, a public face alone should be more than enough.
Characters For Flavor
These are the more fun ones. Your characters may never directly interact with them, but they'll provide flavor and setting. This could be a radio personality, a random person on the street, or anything else. The point here is you want a strong personality that will sell the setting, or some aspect of the setting, to help the players get a feel for the world.
As these characters job is to be flavor and sell the setting, I find that giving a one dimensional but larger than life behavior pattern can work best. This makes it simple to express what part of the world you want to (paranoid, crazy, happy go lucky, etc) and bring it to bear with the characters.
In all three cases you want to use your characters to sell the setting and sell that there is more going on than just what the players are looking at. The more the character is on screen or interacting with the players the more nuanced you want them to be. Some of the characters, over time, you may find are as developed - or even more so - than your PCs are as the party takes an interest in their life and what is going on with it..
Do this right and you sell the world to your players and get them more engrossed in the game than you may think possible. Do it wrong, and you still have a bunch of NPCs to use when you need one. So really, it doesn't hurt. Provided you have the time to put into them.
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