When I'm confronted with a complex scene (like a group of kids in an attic being scared by squirrels), I'll start with a rough sketch to find a composition that has a clear narrative and is (hopefully) dynamic. Once I have the basic composition, I develop the illustration in pieces. I start with the characters and work to make their gestures lively and their expressions clear. Then I tackle the background without the characters. If I don't draw the background separately, I'll be tempted to squeeze the setting in around them - and that tends to look forced and disjointed. Unfortunately, I sometimes draw a background that ends up obscured by a squirrel!