On my current wip, this means about 80% done with the graft and agonizing. I don't plot meticulously and generally start writing on a whiff of direction and a breath of inspiration. I know where I'm going, and I know who I'm taking with me, but I have no idea how I'll get there or who all will surive the journey and what kind of tangled relationship they'll be in if they do.
But once I reach the half way stage, my characters have established themselves, I've learnt what makes them tick, and I'm pretty happy with the path I'm on now after a lot of circling back and rehashing and them bullying me into shape. The rest of the story simply needs to be written with, hopefully, a few surprises popping up along the way.
I guess things would go a lot more smoothly if I started off from scratch with every detail plotted, but that doesn't work for me. Once I know exactly what's going to happen, that's like reading a book after you've seen the movie. It's still enjoyable, there's still excitement to be had, but it's not that same rollercoaster ride of possibilities in the air. My favourite part of writing is making the story take new shapes along the way.