NUMBER ONE MISTAKE: THE LAME CONCEPT
You can write a wonderfully executed script but if it has a lame concept no one will ever buy it.
Concept is the most important element of a screenplay yet it's the most difficult to explain. The concept is the idea you read in the newspaper ads that attract you to go see a movie. When you ask someone what a movie's about they reply by telling you the concept. If it doesn't sound interesting then that's the end of the discussion.
When an agent tries to sell your script he'll call producers and studio executives and pitch them your concept. If it doesn't attract their immediate attention he'll drop the subject. No one wants to push a lame concept.
The concept has to answer two questions:
1. What is it about?
2. Does it interest me?
Many new writers are shy about telling others what their scripts are about. They have a fear that their ideas will be stolen. In today's Hollywood ideas are virtually worthless.
A good idea - well executed - is gold. An unexecuted idea is nothing.
Pitch your ideas all the time. Pitch them to your friends, family, strangers. By their immediate reaction you'll know if you have a concept that is worth investing the time to write into a screenplay.
And don't accept a smile and a nod as approval. Pitch long enough and you'll soon know the difference from mild encouragement and honest excitement.
The pitch is the basic building block of the entertainment business. For some pointers on how to pitch, take a look at Pitching Pointers.