"Give and receive feedback."
Writers fall in love with their story ideas, characters, dialogue, or scenes. But the best way to test these elements is with an independent reader; that is, someone who isn't familiar with your script beyond a logline and, if possible, the scope of your work. Naturally if you reach out to another writer and ask for their time to invest into reading and reviewing your work, then you MUST return the favor. Of course, this leads to the good and bad outcomes, but needless to say: give and receive feedback whenever possible!
1. DO: Ask politely and offer to return the favor. Then follow through!
1. DON'T: Agree to read in return, then fall off the face of the Earth. Not a good way to network with other writers. You'll be blacklisted in cyberspace quicker than you can say, "Superkalifrag..."
2. DO: Give fair, constructive criticism when reading a writer's screenplay. Use the feedback form provided in the articles section of this website. It will help you provide a thorough review and focus on the important elements of the script: concept, story, characters, dialogue, and writing style.
2. DON'T: Consider this an opportunity to sabotage another writer's efforts. And don't make the feedback you provide personal. The writer may not know you - all they know is what you've read and, hopefully, understands that it's just one perspective! But if you make personal attacks because you vehemently disagreed with the feedback you received, then you've wasted your time.