For example, today I'm working on the cover for the next chapter of Digger, and I had a layout all laid out, figure front and center, and I got the background about half done and thought "Hey, that's a really neat bit there!"
A really neat bit.
You know, a whole painting of neat bits like that would look really cool.
And I started doodling, and suddenly my Digger cover is probably going to be a landscape. A nifty landscape, hopefully, but a totally different thing than my original idea. And because I have a fairly strict schedule for Digger, it has to be done tomorrow.
Try this shit on most art directors at that late a stage, and they'll break your painting arm, and furthermore, they'll be fully justified in doing so. There's a coupla people who can get away with it--you and I probably aren't them.* But because I am calling the shots on Digger, I can decide arbitrarily to do this. And it's cool. And man, some days, I just have the greatest job in the world.
*Well, I'm not, anyhow.