One of the first lessons GMs are taught, one of the cardinal sins of GMing we are supposed to avoid, is making our NPCs outshine our PCs. Today though, I want to talk about how to make your NPCs matter. By this I don't mean making them better than the PCs, or more important, or outshining. I mean, specifically, how to make the players feel like the NPC is a person worth valuing - at least as a person/life. With this you can mark out more significant NPCs, or at least the ones close to your PCs and hopefully get something a little better than "he's an NPC, sacrifice him" from your PCs when things get tense.
It Won't Always Work
Before we go into this understand that not all players will be influenced by this. Some people have a very steep divide in their head between PCs and NPCs, and NPCs will never really matter to them. This isn't a problem, but it is something to identify. No point baiting a trap with bait the prey doesn't want, right?
A Name And A Quirk
And there you have it. That's the trick. Give your NPCs a name - a first and a last name, mind - and give them a quirk.
It sounds simple, but the two things work together to make a person. Who do you think is more real "the heavy gunner from squad three" or "Mary 'Boomer' Belle, Squad Three Specialist"? For most of us, Mary Belle is the more real and by the NPC having a name the player will take note. After all, not many people in your game world have names, right?
The fun thing is you can even range importance of NPCs. A first name for people who need to be referenced but aren't super important, and a first and last name for the important people. I recommend just going full bore for all of them, but that's because it makes them all feel that same level of real. It also makes it harder to tell who is important and who is just a reference...making them all more important.
Quirks on the other hand are things that give us character. They're little oddities that stand out. Little touches that maybe are annoying, maybe are nothing, but they make someone memorable.
Quirks can be anything. Maybe the character smokes. Maybe when they count or do math they do so in their native tongue. Maybe they twirl their hair around their fingers. Maybe they have nightmares. Maybe they chew with their mouth open. Maybe they cut a notch into their gun for every kill.
You can find quirks by watching people around you in the real world. You can find them by watching movies and TV shows. You can find them anywhere. You can even just invent them.
Reinforce. Reinforce. Reinforce.
So you have a name and a quirk, now you just need to reinforce it with the players. Always use the name when referencing the character, and bring up the quirk when you can as well. Reinforce them both in the player's minds as real things that are out there and doing things.
This can be as simple as letting a player know when she goes to find Mary that Mary is outside smoking a cigarette and looking at her family pictures. Alternatively it can be as easy as just describing a reaction to a question like "Mary twirls her hair aorund her finger while thinking, then says..."
Keep both in play, and enjoy the rewards. It can make for more interesting and real feeling NPCs.