One issue that I see in a lot of scripts is people writing things that the audience won't see or hear, such as what a character is thinking or the reasons why people are doing things. It’s easy to make this kind of mistake because this is a natural way for people to talk and also to write just about any other kind of prose.
Here are some examples of the kinds of things that can easily sneak into screenplays, but which can’t be seen or heard, each followed by alternate wording:
1a. Josh rushes across the room to check his voicemail.
1b. Josh rushes to his message machine and presses Play.
2a. Rhonda can’t look her mother in the eye because of her overwhelming guilt.
2b. Rhonda keeps her eyes downcast.
3a. Bill and Karen walk through the park where they met back in the fall of 2003 and then sit on the bench where they had their first kiss in order to work out the problems that they’ve been having in their relationship.
3b. Bill and Karen sit on a park bench.
We need to talk.
One time when it is okay to write things that the audience won’t see or hear is when you’re first introducing a character and want to describe his personality so that the actor and reader will get an idea of what kind of person he is right from the start. For example, you might describe a character as, “far older than his 15 years,” or “always bursting with energy and enthusiasm” or “trapped in the body of a white girl.” (Snooki, I am looking in your direction.) But other than character introductions, it’s always best to stick with writing just what the audience can see or hear since those are the only things that will end up onscreen.