Basically, you can do things in the world to improve relations with someone whenever you find them (help them fight zombies, give them something that they want at the moment), and in return they'll act differently. Maybe they'll be willing to trade, or help out with something, or give whatever they have at hand or at base.
But given how the AI is gonna work, I'd imagine situations where you'll come across an NPC who says "I've been starving for days. I'd kill for som food." and you can give them food and they'll be happy. But if you don't' give them food right now, they'll still be off doing their own thing, and maybe they'll find a bunch of food some other way. Then when you meet them again, you can't really offer food to increase that friendliness any more.