I'm up to 12,000 words in a new Viking story and already I'm altering the synopsis to keep up with the way the story is going. Is this bad? I don't know. I read on blogs that some authors do full ten-page character analysis charts or use ennegrams and even horoscopes prior to starting a work, while others just sit down at their computer and think, "Ho hum, let's see what happens today." Frankly, for me, that would end up in a whole lot of trouble!
Those who advocate all the preliminary work on characters say you must have the characters clear in your mind before you begin, because the characters drive the story. I agree with the latter half of the sentence, but not necessarily with the first. I start with a rough outline in my mind of where the story will (probably, unless I think of something better along the way) go and who the characters are, but beyond that, they (and I) discover each other as we go along.
Sometimes the characters just don't react the way I thought they would, and rather than force them into a mould they obviously don't want to go, I feel happier bending the synopsis around them. The changes are not huge. The goal will still be met. Only the route will be different. After all, you don't get to know a friend all in one huge gulp, do you? So why expect to get to know a character before you test them in different and possibly vexing situations?
(Sometimes I feel talking about characters I'm inventing "not wanting to go where I want them to" is nonsense, even pretentious rubbish, but that's another topic for another day!)