As writers, we play God (or Goddess, as it were). It's in the job description. We play God(dess) and, if we want to write a good story, we often have to flex our divine muscles to change probability to something a little more interesting. In speculative fiction in particular, we're divine--we have created a world.
But, there's something worth remembering amidst all of that worldbuilding we lovingly do: you have to follow every thread out to the very end, at least as it applies to the elements you're dealing with in your book.
I'll give you a recent example form my own work. I'm currently worldbuilding (mostly culture building, at this point). The culture is designed to deter prejudice and dishonesty: everyone wears full body veils, covering everything but the eyes, and rings that signify socioeconomic standing, profession, marriage status, and number of children. You can't lift the veil in public.
Here's the issue: I'm writing a very strong romance thread in this book. So I had to go in up to my elbows and figure out a new system for public displays of affection. Then I cam to the realization that I had no way for unmarried couples to eat or drink in public, since they would have to lift the veil off of their face and risk showing themselves. So I had to design new chairs.
These were all issues that came up because of one relationship. Between two characters. Lord only knows what else I'll find before I get to the writing stage.
When you write a totally invented culture, it's necessary to think it all the way through. Even the smallest difference between their culture and ours can cause huge ripples, and you don't want to ignore them. Somewhere along the way, someone will go to the fridge and, while looking for a beer, figure out that something in your book doesn't make sense. Okay, maybe it's not a sure thing, but are you willing to take the risk?
Embedding myself in culture,