3. Have Something To Say
There are a hundred different reasons somebody might want to write a screenplay, but one might argue that the best reason to do so is because you have something to say. Many writers associate "having something to say" with serious movies, but that's a fatal mistake: every movie that gets made should have something to say on a least one subject. Doesn't matter if it's
Schindler's Lists or
Transformers: why is this story being told? What is the point? And movies that fail to do this fail to justify their very existence. The best movies will give you something to cling to once you've left the theatre... a reason for having taken your money. That's to say, write something that has worth and - ideally - hasn't been done before. If you're writing a parody of B-movies, make sure you've got something fresh to say about B-movies. Even
Machete had something to say about American/Mexican politics, after all.