yes the writer does have a certain influence, but again, to force that influence would leave an effect much like God doing absolutely whatever it takes to get me on that plane regardless of the fact i would rather be butt-raped than fly. in order to make the story work, you really do have to relinquish a certain amount of that control, or else get backed into or corner, or worse, produce a shitty story.
do i believe that a story can ONLY go one way? of course not. stories reflect reality; in reality, we always have a choice. same with writing. there's always a choice. but one of them is always WRONG. does that mean it CAN'T be done? no, but as experience in life teaches us, doing what we shouldn't holds disastrous consequences.
I really do believ that to a large extent, a writer can't know how the tale will evolve. yes, he can know that Bill is 6'2", 198 lbs, blue eyed, green-haired, reminiscent of an overgrown elf of Tolkien imagery; he can know Bill's going to have some serious struggles, he can know Bill's mom is nuts and his dad bled to death after being castrated and hung from the barn roof by gravity boots, he can know Bill's dog is gonna get hit by a car and that it will profoundly affect Bill. What he can not know is how it will affect Bill, or how much Bill loved or hated his parents, or that Bill's going to die. To assume he does know such things brings an element of profound doubt into everything that follows.
So...yes, I concur, it's a bit of both. To say, for instance, that Steve had no idea Georgie Denbrough was gonna die in It...well, come on,that was one of the crucial plot points, and Georgie, although crucial to the tale, was not an essential character, but rather a sort of forethought that would profoundly affect/alter the lives of each crucial character.
On the same token, to say he knew from the get that Stan was gonna slit his wrists...no. I mean, sure, we all know the type of guy Stan turned out to be, and how much sense his suicide therefore made, but to assume Steve himself knew it at the novel's outset would be a bit preposterous. After all, the biggest truth pointing to the evidence of the original question in this thread is precisely this:
writers, as well know, write for themselves. whether they get paid for it or not, writers write. that being said, answer the question, if the writer already knows essential plot points and can therefore successfully predict the outcome of the finished work, why in the fuck would he bother writing about it at all?