I usually cast myself in things because acting is how I best relate to artistic impulses. It's what I've wanted to do since I was a child, so a scene usually plays itself out in my head with me performing it. And if I cast myself that's one less person I have to pay, one less person I have to explain my vision to, one less person I have to worry about.
The money for my movies mostly come from talented and generous friends willing to work for almost nothing. As a producer, my job is to get as many people to give me things for free as I could. And most people are kind, and they know how hard this kind of thing is, so they are willing to help how they can, and reduce their fees how they can.