Writing a long story, a novel say, is like taking a world spanning journey but ending up in a box canyon. Or maybe it’s like walking an unknown trail but every so often you fling out a spool of thread letting it drag along behind you.
The point is, when you start writing a book you have the wide universe open to you. But as you go along you create these tendrils of story dependency that you must remain loyal to. Every new thread ties you to a logical canon you must not betray (or forget about or violate, etc.)
Say you want to write a story about a young man who meets and influences a famous person in history. So this person starts off as a child slave in Rome circa the year 500 CE. He’s got a limp from when he was very young, a wagon crushed his foot. But his eyes are lovely and his smile, a shimmer on a clear pond. His mother was a slave to a kind senator whose wife was ugly and hated the boy’s mother for her dark-haired beauty.
You see how easy it is to create a world where before nothing existed.
But let’s say that 50,000 words later this boy, now man, needs to make a journey on foot. Well, remember that he’s crippled, so whatever road he takes he must be crippled the whole way, it must influence his entire trip. He meets a woman who reminds him of his own mother, dead now for four years. Oh yeah, what did his mother look like? And who was that senator he slaved for as he can now mention his name as leverage? And what did that senator’s wife say to him when he was twelve, caught stealing honey, and viciously giving him that scar beneath his right arm?
A wide universe eventually turns into a box canyon at the end of the story. All those early decisions become a log of canons that cannot be violated. You must remember every one. Sure some will become natural characteristics of the story but others will be these threads that tie you to the core path of your plot. And tighter and tighter they bind you until at the end, you’re like a worm trapped in a cocoon and the only place left to write is that hole at the end of your tale.