One thing I sometimes hear from writers is the idea that characters have minds of their own. The author doesn't tell the characters what to do, the characters just do it because that's who they are, and then the author just tries to describe it.
I definitely do not feel the same way about my characters.
me, the characters are not set in stone, and can be changed to conform
to whatever actions I want them to perform. I ask myself, what kind of
person would do that? And then that's who they are.
Perhaps I'm too early in the novel, and the characters' choices will become more constrained later on. Or perhaps I'm just doing things wrong. That's always a possibility.
There are disadvantages to building the
characters as you go. It encourages flatter characters. It may lead to
facades that only make sense from the perspective of the particular boat
ride created by the book. If you were to step out of the boat and see
the cardboard cutouts from the other side you'd see that there was no back story, no additional character details, nothing.
I would, however, defend my approach. I believe it mirrors how real people behave. First they act. Then they come up
with justifications for their actions. And finally, they become who
they think they are. Choices and behaviors make up who a person is. So-called "character traits" are only descriptions after the fact.
I also think that when some authors have a whole coherent character in mind from the beginning, maybe they're just thinking of an archetype. We have all these ideas and prejudices about what kind of character traits go together, and I think it's an illusion. People have a mostly random combination of traits, and we just have an illusion of a coherent whole.