A novel is like a tree...
What usually happens, at least to me, is that I got some leaves here, some leaves there, and I start from that
It is a mistake to think that you can see the whole tree before setting off to write
Then I got already a bunch of leaves, maybe a part of the trunk, some things very well defined, somethings quite blurry
Eventually you start getting a sense of what kind of tree this is. Another mistake is to think that you got the plot first, what usually happens is that the plot gets build around the connection you make between all the leaves you already got.
As if I were painting some fresco. You go on the left, then up, then to the right, it is not realistic to say you do it in order.
With characters the same thing happens, you have one action, another, another, one event, another, another... The character ends up being the ending result. And then a starting point.
Orhan Pamuk Advise for young writers