"Always flat and unfeatured to begin with"

When I try to write a story with no inspiration and no notes, the characters come out flat. There's little to go on to move things forward in an interesting direction.

Apparently this is how things start even for Stephen King. From On Writing, page 164:

The situation comes first. The characters—always flat and unfeatured, to begin with... come next.

How can that possibly work? I have no strong opinion on his writing — I've only heard The Green Mile (pretty good) and only read The Gunslinger (bizarre) — but loads of people like his work. So evidently it does work. But how?

King continues:

Once these things [situation and flat, featureless characters] are fixed in my mind, I begin to narrate. I often have an idea of what the outcome may be, but I have never demanded of a set of characters that they do things my way. On the contrary, I want them to do things their way.

What I don't understand is how they can possibly do things their way when they start out "flat and unfeatured to begin with." Obviously it works out for him. I haven't found it to work well when I start with literally nothing. I don't find that my characters rise to the occasion; rather they seem to droop, as lifeless and lackluster as the effort I've put into them. But I've also never had the patience to fill out a big Character Notes document outside of fanfiction, and that seems slippery to even get started with for similar reasons.

Character has got to start somehow. But how?