This could be considered a writing post, but I like to think it’s a little more universal. You could, for example, use it while creating a D&D scenario for your adventurers, or making a film, or building a video game.
I’m talking about worldbuilding. In particular, I’m talking about building worlds where you’ve set the rules. (This is different from Chekhov’s Gun. I’m not talking about foreshadowing.)
When we create worlds, we make decisions. For better or for worse, we have to live with those decisions. If the naming convention for your elves requires that that all have a glottal stop in the middle — ie: Tel’iv — then you cannot name one of your elves Bob. B’ob, yes, but not Bob. (I warn you right now, keep that in mind. By the time you’ve finished typing out the forty-seventh elf name with an apostrophe it’s safe to say that you’ll be willing to throw the next elf against the wall and inform him that hence-forth in the land of men, he shall be known as Bubba.)
This is where the rules for magic come into play. If you have created a ritual magic system, even the most basic spell needs a ritual. (“St. Ant’ny, St. Ant’ny, come around, something’s lost the must be found.”) You might say that all magic rituals in your world require a poppet. That means that your magician needs to carry around a bundle of poppets in her bag. Compare this to the wand-based magic in the Harry Potter stories where a single form and word can create a spell. Let’s do one more — Harry Dresden uses potions that sometimes involve the smells — like home-baked cookies — or sunlight, which means that Dresden (let’s not get confused) can capture sunlight and store it. Potter uses plants and animal products, but not intangibles. Different magic types, mean different potions.
If you work in the science fiction realm, and your create a species, then members of that species need to follow the rules you’ve laid out. If you have a Betazoid, s/he will be an empath. Even a half-Betazoid, will have some empathic traits. It’s a dominant gene. If you have a Klingon, s/he will be adverse to Tribbles.
On the other hand, you may create societal rules for a species, those may be broken — as Terry Pratchett has with his dwarves. It is a given in dwarf culture that any female who wishes to work, will present as male to the world. However, when a female dwarf decides that she wants lipstick and a dress, she can modify that societal rule. (There are always consequences good and ill to subverting a societal rule. Remember that too.)
When you don’t follow the rules you’ve laid out, that’s where you get into trouble. This is the ranty part of this topic, where the idea all began.
Example: In the first few episodes of Voyager, it was stated that Kes’ species only lived for seven years and that she was three. Her boyfriend (a different alien species) knew he’d outlive her, but was willing to make it work.
First thoughts: Wow! That’s brave. Let’s show how aging can affect relationships. Let’s show how the compressed life expectancy changes cultural pressures for breeding. How will the crew deal with her inevitable death.
Reality: The writers changed the rules and said that suddenly, because she was in space, she wasn’t going to die.
My reaction: RAGE. RAGE. WTF! Don’t be cowards.
Also, if you decide that magic has a certain cost — calories, mana, premature aging — stick with it. Jim Hines’ Snow White ages prematurely every time she performs high-level magic. She makes the choice to make that sacrifice for her magic. If your magician can’t wear clothing that contains polyester because magic makes it melt? Give me a scene where he bitches about men’s fashion and the cost of pure cotton flannel shirts.
So, in summary, make a rule. Stick with it. And make sure you know the rules going in or else you’ll never make it out alive.