So, while watching Skyline recently, I got thinking about motivations - and how the ones in the movie just don't make sense.
Trust me, you won't care. This was a big-budget, low-quality blockbuster a few summers ago. It's pretty rotten, has a cast all plucked from a variety of mediocre TV shows, and has a lack of coherent direction that would make McG proud.
It revolves around the story of a group of people trapped in a high-rise as aliens attack, and begin sucking people up into their ships like great big Dysons. As it turns out they're biomechanical/organic aliens, who use human brains to run their drones and war machines. They steal them straight out of people's heads.
You're trying to tell me that, despite having organic technology far in advance of ours, (presumably) some form of faster-than-light tech, and a whole different, you know, DNA structure, these guys can just plug-and-play human brains into their stuff? And they can't find a better source of brains (like cloning) or, better yet, build a computer that can do the job a thousand times better, faster, and more reliably than a human brain can?
Now, if this was a campy, fun sci-fi movie, I wouldn't have asked. I can accept a lot of suspension of disbelief when a film is knowingly being silly. But a lot of effort is made to make the aliens seem threatening and alien, to show the real horror of an alien invasion, and to ratchet up the freaky factor wherever it can. So when it asks us to swallow something so dumb, it jars pretty hard.
The Matrix movies had a bad case of this type of plot hole - using humans as thermal batteries. So, you feed the human 1kg of food, and it makes X amount of heat. As we're not a perfect machine, some of that heat is lost. Wouldn't it be easier for the machines to burn the food, gaining a far better ratio of energy in:energy out, than have to have those pesky humans all over the place? They shit, they breathe, they can only survive certain temperatures... you're far easier just keeping a stock of fuel than using humans for it.
These motivations make little sense, from an outsider's point of view. So, when running a game, make sure your villains (and other NPCs) have understandable motivations.
Take the Necromancer - a classic Bad Guy. Why would a Necromancer hire living minions when he is shown to be able to make far tougher, more impressive, threatening, and useful minions from their corpses? Many would think he's wasting money, and it shows a lack of forethought.
So, give him a reason - perhaps he can only control so many Undead before his power is too stretched out. Maybe he values their ability to think independently. Maybe it is cheaper to hire mercs than make zombies (and, when they die, you take their money back and turn them into zombies! PROFIT). Maybe he just likes them as people, and gives them the chance to join him instead of killing them.
Or the Big Bad Overlord - why does he concern himself with the activities of a bunch of low-level adventurers? Maybe hes paranoid, maybe his advisor is an Oracle, maybe he has a soft spot for one of the PCs. Maybe he's the father of one of them!
Why doesn't the city send the guards or militia to clear out dungeons, instead of hiring mercenary adventurers? My favourite answer for this: taxation. If the Duke were to use his Duchy's resources (technically, the King's resources), any treasure and magic items they found would be taxed (on the King's special Dungeon Tax of 35%, no less). However, if the adventurers went and found gold, then came back and spent it in town, increasing the flow of cash, and only being charged standard tax! It's a win-win!
Whatever you do, make sure he has a reason for doing it. Even if it's based on information that is wrong, it has to make sense to the person doing it - otherwise you end up feeding people too much and then trying to plug their brain into your laptop.