Neil Gaiman:

Whenever I do things because I want to do it and because it seems fun or interesting and so on and so forth, it almost always works. And it almost always winds up more than paying for itself. Whenever I do things for the money, not only does it prove a headache and a pain in the neck and come with all sorts of awful things attached, but I normally don't wind up getting the money, either. So, after a while, you do sort of start to learn [to] just forget about the things where people come to you and dangle huge wads of cash in front of you. Go for the one that seems interesting because, even if it all falls apart, you've got something interesting out of it. Whereas, the other way, you normally wind up getting absolutely nothing out of it.