I need to know the key scenes. They're the pillars the story is strung between. This is something akin to Jim's "Big Middle" model excpet with more points of attachment; for a novel of 100,000 to 200,000 words, which is the range my stuff tends to fall out in, there are usually five to seven of these. (The current work in progress has thirteen, but that is because it is insanely long and would be three fat volumes or more slimmer volumes - it falls into nine internal parts, and i have no strong feelings about how those are divided up with extra bits of cardboard - if it ever saw print.)
One I have the key scenes, I can audition for characters. See who I've got in my head who will react to key thing X by doing Y to keep the plot going, and then figure out who they are, building from those character moments and the world they grew up in. (I do need a world, but worlds are much easier than stories; I have worlds falling out of my ears.) Make sure to stop Aramis from sneaking in under some assumed name. (Never work with Aramis, because he never tells anyone all of what he is doing, including the writer.)
And then, well, I have people, i know where they come from and go to and the key bits they're going through, the rest is working out the path from pint to point to point and then seeing where I can put cross-connections, as many as possible.