There are few things worse to a reader than someone who has written a screenplay solely for the purpose of trying to relay some kind of urgent societal message. Writers like this usually try to beat people over the head with their moral agenda as if it were some kind of blunt instrument, which is very reflective of how readers feel after slogging through one of these screenplays. They’re a pain to read and invariably cause at least a little bit of resentment because no one likes being preached to (especially about ending sentences in prepositions).
The perfect example of this came from the 2003 Academy Awards show. When Michael Moore won Best Documentary for Bowling for Columbine, he turned his acceptance speech into a direct attack on President Bush for invading Iraq. The vast majority of the audience probably agreed with him, but they reacted with boos because he was being such a yammering prick about how he delivered the message. Contrast that with Adrien Brody’s speech later in the evening when he won the Best Actor award for The Pianist. He thanked people graciously and then shared his heartfelt wish that everyone stay safe during that time of war, especially a close friend of his who was just deployed to Kuwait. That speech got across a very similar message as Michael Moore’s, but everyone applauded Brody and felt genuine compassion for his friend, rather than feeling resentment for being subjected to a grandstanding sermon. It probably didn’t hurt that Brody had just kissed Halle Berry in front a billion people either.
I’m not saying that you shouldn’t have a message in your script, because the opposite is actually true. Every script has to have some kind of message or should at least perform a thoughtful investigation of some universal question, but you should never deliver that message in a heavy-handed way. If you slip the message in under the radar, your reader won’t be able to resist it and will feel exactly how you want him to feel. If you barrage the reader with your message by having your characters talk about it directly and at length, the only message you’ll be delivering is that he should pass on your script. I probably could have said that more subtly.