At least, that goes for the major players. Regina and the Assassin, for instance, or Tip or Marta or Charlie. These are, after all, the ones I really spend time with: not just the narrator, but the people whose lives are intertwined with that narrator's and who I must deal with in just about every chapter. As in real life, being with them and trying to get to know them over such a long period makes them (relatively speaking) easy to write. I know them. Sometimes they surprise me, but in general I can tell you how they will react in a given situation. I know Regina has no use for Dickens. I know the thing Tip misses most when he's miles from land is the trees. After a while, I just start finding these things out.
minor characters are a different matter.
Just about every story has them - people who exist on the periphery of the story, whose interactions with the protagonist are infrequent, who have a part to play but do not sit at the heart of the tale. I don't know about you, but I find these ones the tough cookies. When you have a fellow who drifts in and drifts out, how do you get to know them? How do you make them memorable? How do you make them a person?
I've read some tips that advise writers to "give the character a defining trait" - a drawl or a habit or a word they're particularly fond of - to make them stick in the reader's mind. I suppose if that is the goal, it's a reasonable approach, but it rather smacks to me of "tagging." You scribble down the defining trait, punch a hole in the card, put a string through it, tie it to the character's leg and voila! An easy something for the reader to note! No great brainpower needed on either side of the equation, but the reader remembers (a little remotely) who the character is and so it's all good.
Note, I'm not at all saying that there is something inherently wrong with defining traits; I know there are several in my own books, and I also remember a novel I read some while back where a character habitually said, "Listen now," and it worked. It fit him. It was natural, and I liked it. I'm just not a fan of the dartboard approach to writing - picking a trait and plastering it to the character without even a by-your-leave. A quirk doth not a person make.
And that, I think, ought to be the goal with every character, major or minor: to make a person of them. They won't all be equally vibrant. My word, some of Dickens' heroes were downright pale! But we ought to do justice to the characters in our stories, and not make caricatures of the poor fellows.
I still find this a tricky business, but one thing I've found helpful in the process of writing Tempus Regina, whose cast is larger than any I have dealt with yet, is to take the time to dig into each person's backstory. Not that it comes out in the actual novel, mind, because that would be downright tedious. But when I feel a person is flimsy, it is helpful for me to take them, go back into the bits and pieces I know of their past, and write them a short story. Casting them as the point-of-view character forces me to study their thought-patterns, and following the snatches of their undeveloped history gives me something to work with. Then, when I go back to the novel and pick up where I left off, I think, "Nyaha! I know who you are now." And even if what I wrote ends up having no bearing on the plot, it still gives me confidence and grants that character that much more reality.
but sometimes what I write does have bearing, and that's even better.