Something that you should take particular notice of is the fact that the best scripts have very few explanatory passages. Adding explanation to the descriptive passages of a screenplay is the most dangerous trap you can fall into. It's easy to explain the psychological state of a character at a particular moment, but it's very difficult to describe it through the delicate nuances of action and dialogue. Yet it is not impossible. A great deal about this can be learned from the study of the great plays, and I believe the 'hard-boiled' detective novels can also be very instructive.