Say you're playing a game set in WWII. You and the troops have been battling your way across Europe. The going has been rough. To get health points, you've got to eat the slop that passes for food in your rag-tag regiment. (The war's been dragging on, and both the tent lodgings and the food look progressively less appealing.)
Let's look at some the dialogue of the Cook, a minor NPC. First we'll critique a weak example, and then consider how it could be made interesting.
An Example of Weak Dialogue
You approach the Cook, who's serving food. He says:
COOK: Here's your food.
You might say that you'd never write a line like that. Maybe not. To my ears, however, far too much NPC dialogue hovers at that level of artistry.
How can you lure people who have become accustomed to hearing writing in films and television into your game, if they're going to have to suffer through lines like this?
I call this kind of writing "robo-speak," because the dialogue might as well have been spoken by a robot. It reveals no discernable personality by the speaker. Have you ever met someone who has no detectable personality? Probably not. Thus, robo-speak breaks emotional immersion, because it's not realistic.
Believe it or not, I have actually encountered such a person. She's an elderly member of my extended family. In my entire life, I have never once heard her offer an opinion on any subject or color a sentence with any kind of opinion or slant. If you told me that she was a Pod Person, I'd probably feel relieved to finally have an explanation.
The bottom line, though, is that she's hard to be around, because there's no "there" there.
So NPCs who only talk in "robo-speak":
Let's see if the Cook's dialogue can be improved upon.
You walk up to the Cook to get your meal.
COOK (concerned): They said you were dead.
At least he has some emotion; he expressed concern for you.
COOK (re: the food): Eat it -- before it eats you.
He has now shown at least one Trait. Let's see if we can give the Cook two Traits:
COOK (pleasant): It's probably chicken.
Here he's both Pleasant and Ironic. Another example:
COOK (apologetic): It's bad, I know. But hey, at least you're alive to eat it.
In this example, he has two different Traits. He's Caring with a sincere desire to feed the men well, and he's a bit Cynical.
You probably noticed that I needed to extend the length of the dialogue in the last example, possibly to an unacceptable degree, to get in the second Trait. The more Emotioneering you cram into a single line of dialogue, in general, the harder it is to keep that line brief.
Things get more complicated when the dialogue needs to not only add color, but to also prompt the player to take action. Most NPC dialogue prompts player action or gives the player important information.