That’s a not uncommon approach among CoG writers, either, to judge from some past conversation threads. Here’s what I said on one about a year ago (engaging with a fellow forum member who, unlike you, Stewart, was using stronger words than “bizarre/foreign to me” in response to the my-characters-steer-me framing):
Funnily, since writing that, I’ve been finding that metaphor more and more apposite in my work – much of what I’ve written in the past year+ really has felt more like discovery than design. I basically never get a whole character formed at once, and until I start writing them I usually have only a general sense of the kind of person they’re going to be. More and more, I’ve found myself surprised by what comes bubbling out when I start that process, and how it takes things in a rather different but truer-feeling direction than I’d initially expected.
On other questions… There’s some part of me in just about all my characters, but almost never the most important part of them. Many of them are much more shaped on people I know, or have observed, or have read about.
And I definitely don’t go down the route of writing their birthdays and favorite music and everything about them in advance. As features of the character become relevant to the picture I’m painting in the story, I come up with them, not otherwise.