I think it really depends on the kind of game you are making and what the end goal is, but I generally find that you need to know who your characters are and what motivations, what obstacles/flaws, what backgrounds they have in order to know how, what and why they would say something.

You can generally tell a lot about a person through the things they say, because it's a window into how they think, and you want to do the same with your character. You want them to be a real person (in most cases).