When NOT to write

Have you ever read a novel that started off one way and then somehow morphed into
something totally different mid-book? That kind of stuff throws me as a reader.
But as a writer, I can see how it happens.
You start out with a bunch of newbie characters, then you put them into difficult situations.
They react, and how they react shows what their true character is like. Before your eyes, your
little newbies are growing up (*sniff, sniff!) having opinions, growing a backbone (or not) and becoming
themselves.
At least that's the way it is for me.
Other writers might experience it differently. Maybe their characters come to them fully-formed but then the further they go along in the novel, the characters end up changing or going in directions the author did not anticipate.
This is why, when writing a novel, some 'writing days' are spent reading.
Today was a reading day. I hadn't planned on it, but I'd gotten to a place where I needed
to be reminded of where my MC had been and how far he'd come.
Writers have to stop sometimes and read from the beginning -
to make sure Main Character in Chapter One matches Main Character in Chapter Twenty-One. And if he/she doesn't, you have some 'splainin to, Lucy.