I've been developing a secondary character for my new novel, Keeper of the Pond. He's only in the first half of the book, but the man himself, and what he does for a living (he's a magician in the 1930s) will have a big impact on my main character and the choices he makes later in life.
This is my favorite part, creating the characters that will carry the story from beginning to end, letting them get under my skin, allowing them to become familiar enough that I start thinking about them all the time.
Want a character to be shorter? Poof! He's now four feet tall. Think she should have an addiction problem? Tap-tap-tap on your keyboard and suddenly the woman's gambling online after her family goes to bed. Need some dramatic tension? Poof again as a door opens and there stands the 15 year old son your protagonist never knew he fathered back in high school...
And all this states the obvious: that writers really are just a buncha control freaks, don't you think?