One of these convenient devices that I see far too often is when one character overhears another person saying something important. How many times have you actually ever overheard something in your life and, of those times, how often was it something that changed your life? Of course, this happens every once in awhile, but only about 1/1000th as often as it happens in screenplays.
Even if overhearing critical information were commonplace in reality, you still wouldn’t want to do that in your script because drama is always best when it plays out face-to-face. When your characters actually interact with each other, we get to see the drama unfold between them, not to mention the consequences that the first character would face for sharing such critical information with the person who can use it the most. That’s all really good stuff to put in a screenplay – much better than showing one character leaning against a doorway as another one prattles on carelessly about some sensitive topic. If you don’t show the interaction between these two characters, you are robbing your audience of a dramatic interaction at a crucial point in your script. If you have any overhearing in your script, try to re-imagine that scene as taking place face-to-face and you’ll see just how much drama that can add to your story.