Dan Dos Santos (who is a brilliant artist) just started up a blog with some other power artists. His thoughts:
Sex and violence? No problem! I've done a lot of that in my covers. But capturing humor was something that never came up, and quite honestly, scared me a little. This wasn't slapstick or something else that could be brashly depicted. It was a subtle campiness that pervaded the book, and really brought a lightheartedness to an otherwise intense story. Capturing this flavor was something I had little experience in, but I knew it was essential if this were to be a good cover.
After struggling for a while with the initial sketches, I ended up finding inspiration in movies like 'Army of Darkness' and 'Men in Black'. It occurred to me when watching these movies that I never actually feared for the safety of the heroes. I knew they would succeed, no matter how ludicrous and over-the-top their situation was. Why did I know that? Because they knew it. And therein lay the solution to my problem... stupidity. Nothing says 'campy' quite like an imbecilic lack of fear in the face of obvious danger. The more danger there is, the more fun it is.