It starts off like this:
You'll see I make some notes along with whatever I sketch (can we call these sketches?)--these notes often include characteristics, what they may wear, etc. I drew this little guy with a space-like backpack, and a sweet ray gun. Big eyes, even bigger ears. And yeah, I wrote "pew pew" coming from his ray gun. Nice, huh?
I call him Dr. Phineas the Fennec, aka, Finn.
I then create a Character Bible, which is a blueprint (along with the wicked sketch above) for my artist to build from.
I included several links--I always do this--for the artist's reference. (If I'm creating an actual human being, and not, say, a fox, then I'll often "cast" the character as a famous celebrity.)
There was clearly a distinct look I was going for and the very talented Erik Thompson nailed it. The radness of Noodle from Gorillaz with the mischievousness and sciencey-ness of Dexter from Dexter's Lab.
And here's what I got less than a month later.