Would like to mention. The Art of Dramatic Writing by Lagos Egri.
He has a very simple effective way of developing characters. Then he takes plays by Henrik Ibsen and other writers and breaks down the dialog to illustrate his theory.
Really brilliant, changed how I write completely.
It is a free writing software for several purposes, writing comic scripts is one of them. There are little things you can buy to upgrade it, but i dont think anybody needs them. And IF you think you need them there is a pack that is like 10$.