I've always known that if I recorded an album, it would come out, and people would enjoy it! Whereas if I wrote a movie script, chances are better than even that I'd just be another guy in L.A. with a movie script in his drawer.
It's hard to really articulate what the parameters are that make one song parody-able and another song not, but if I can come up with a good enough idea for it, I go for it, and if not, then I have to move on.
Somebody will come up to me after a show and have me sign their arm, and the next time I see them my autograph has been permanently inscribed on their arm.
My process for the parodies is that I get an idea for a song and then get approval from the artist and then go in and record it and probably try to get it out as soon as possible.