Why do fictional characters taken on a life of their own? How do they do it? They're make-believe, right? They shouldn't be able to do that. When writers say stuff about characters deciding on their own lines and adding levels to their own personalities, I tend to huff and puff about affectation and literary arty-fartiness.
But I've just had a germ of an idea - this in itself is a rarity. Well, more than a germ; I knew exactly what I was going to write. And it's taken on a life of its own. The minor supporting strand has become the main feature. The main plot's been shelved for another day and, even at that point, maybe some other idea will barge it out of the way? (Given that the main idea heavily featured grumpy dustbinmen, maybe that's not too bad a thing?)
It's a bit like automatic writing, aided by some long-gone spirit of a writing antecedent. Well, bring it on!