You would think writing would be easier than real life because of this wide-open, anything goes-ness. But English majors everywhere could spend eons clueing the rest of us into the fact that the fictional universe has to be more logical than the one we live in. Ours has loose ends all over the place. In fiction, everything has to make sense. That doesn't necessarily translate as "happy ending," but the ending has to make sense. When it doesn't, you come away feeling gypped.
You see this most strikingly in fantasy. In all of the fantasy-writing workshops I've attended, the big take-away has always been to keep your universe logical. The magic, particularly, must be logical, since you can't have a spell work one way in one chapter and a different way in another (unless that's the nature of the spell - still a logic). Harry Potter does this beautifully - you can predict with fair certainty that if you see a cool spell in the first chapters of the book, it will somehow save his bespecticled hiney in the last. It's no less important in everyday contemporary fiction, though the "magic" may be a relationship, symbolic object, or family skeleton.
One of the biggest challenges (for me, anyway) is to fit the unknown in with the known. I know my character relatively well by now, and I know how she feels about various people in her life and how the events of the novel are shaping her and those around her. In particular, I'm currently focused on her relationship with her parents, which is probably one of the biggest arcs in the story. Hence the parents themselves, and even their relationships with their own parents, must fit together. I know some of the pieces, and I can guess at others. I can see how they have affected my character, but it's up to me to be the logic detective. Eliminate the impossible, consider the (sometimes ridiculous) probable. Sometimes it helps to try different scenarios on for size:
- Mother came from rich family, married beneath her, always trying to measure up.
- Mother came from poor family, wanted to marry above and didn't, feels constant shame.
- Mother came from intellectual family, married blue collar, must overcompensate.
Any of these could get me close to where I'm going (mother wants to prevent daughter from making mistakes), but that's not enough. I need to hit it precisely. Psychology can be messy in real life, but not in fiction.
Amazingly, these problems often solve themselves as I just keep going. Start with the familiar, explore to the extreme, come back to what is true in this virtual semilogical universe.