Take a look at this excellent piece from Cedar Sanderson over at the Mad Genius’ Club, readers!
Cast and Character
I’d asked in the spiky book club server what topics they’d like to see me touch on in posts, and the first response was to ask about fleshing out supporting characters. Which is a good point. No one wants to wander into a book and see a bunch of cardboard cutouts propping up the joint.
The quick and easy answer to filling out a supporting cast is to remember that each of them is a person (whether human, cat-dragon hybrid, or silicone based lifeform) with motivations and reactions. They will act, or react, based on what is being done in the story. You have to give them logical motions or the reader is going to wonder if they are just part of the scenery – hence the reviews complaining about wooden supports. Why logical? Well, real life is full of mysteries and conundrums, and while sometimes you might be able to figure out the underlying rationale of the people around you, mostly you won’t. In fiction, though, you should give some reassuring clarity to the readers.
A Song of Joy by Caroline Furlong 
That, “No one wants to wander into a book and see a bunch of cardboard cutouts propping up the joint.”, directly intersects my biggest insecurity about the sci-fi/science fantasy story I am writing in fits and spurts; it usually gets phrased as, “Do my characters seem enough like they could be real people?”
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