I'd like to start with a quote I came across that made me think:
"Writing a novel is like driving a car at night. You can only see as far as your headlights, but you can make the whole trip that way."
E. L. Doctorow
It brought up the question of outlining, knowing what happens next, letting the characters or story take on a life of their own, and everything in between that concerns the process of novel writing. I actually think this quote holds true, even if you meticulously outline, sketch characters, and pre-imagine your story before you actually start to write. Because at the end of the day, if you end up following an outline or plan to a T, you'll wind up going exactly where you expect, and the only real problem with that is that it's precisely what your audience expects as well, and they will be bored.
All the planning and the outlining, however, is still extremely useful. No matter if you deviate from it just a bit, or deviate from it a lot, or scrap it all together at some point. Of course, there are those who work best with no plan at all, but in general this can slow down the process of writing a full novel. It's also possible that this will decrease your motivation. I know this is the case for me. I use outlines to keep me on track, not only in terms of the story, but also in terms of productivity. It lets me know I'm getting somewhere and if I have a sense of progress I am more likely to keep going.
Of course, there is always the point where you better stop planning, and start writing. Otherwise, the absolutely most obvious thing will happen, that is, nothing at all.
"It is good to have an end to journey towards; but it is the journey that matters in the end."
Ursula K. LeGuin