Tessellation. It all has to fit together. There's a pattern and a method to it, which I had a lot of fun with last weekend. And another nice set of parallels with writing.
Just because it follows rules on how it is constructed doesn't mean the end result isn't pretty darn individual. And even though each tile has to have the same shape, and relate to the other tiles just so, doesn't mean that the artist can't play around with what's inside them all she wants.
And the artist can break the rules, so long as she does it where it doesn't break the whole. In this case, at the edges.
And you think you have trouble with characters dictating things in your head? The way the angels and demons were insulting each other, I was lucky there wasn't blood shed.
But what, you say, must stay the same in writing?
Well, the World for starters. Even if you're planning on introducing a big tech change, the World must be shown to be one that will gleefully adapt, or reject in horror, innovations. The civ that considers color TV the work of the Devil isn't going to have universal cell phones in ten years.
And unless the character is taking speech therapy, the accents. The quirks of speech. The tendency to rattle on when nervous or to be silent when upset. Mid-book personality transplants are bad.
The rules of magic. And physics, biology and chemistry. If you're going to break those, you need to only play around in the frontier areas, where even the experts have their late night doubts about String Theory or Dark Energy.
What should not be the same? Each character needs to be different. You should not be able to swap the names and have a conversation sound equally valid. The characters will feel differently about the same experiences. Men aren't going to react the same as women. People who are insecure, jealous or nervous won't see the same act in the same way a self-confident, mellow type will.
What can change? The POV, the mood, the pace, the setting of each scene.
What ought to change? The characters. They need to mature. Fall in love. Learn skills, gain confidence. Get beaten up, collect a few scars. Some of them die.
So, in writing, what is your worst problem? Fitting the pieces together properly? Too much or too little change? Breaking the rules in the wrong places?