One simple problem that I see pretty frequently is when people write a description of what a character is saying rather than writing down the words in dialogue. For example, I’ve seen people write things like, “they argue for a few minutes,” or, “they introduce themselves to each other,” or even more elaborate scenarios like, “they discuss the terms of the deal and arrive at an agreement.” The basic rule is that if any one of your characters says anything at all in your script, then you must write down every word in dialogue.
Look at the movie “Shine” in which Geoffrey Rush plays the brilliant but emotionally impaired pianist, David Helfgott. He spends a lot of the movie mumbling incoherently in a rapid-fire blur, but screenwriter Jan Sardi didn’t write, “David mumbles to himself as he walks in the rain.” He wrote down every single word of that long string of madness that came out of his protagonist’s mouth. If he didn’t, then how would Geoffrey Rush know what to say? This is a pretty easy mistake to make, so just remember to watch out for any times when you find yourself describing what a character is saying and then change that into actual words of dialogue instead.