How To Write Action Lines In A Script . The line begins at the start of a shot and the line ends when the shot ends. That could mean either action or dialogue has been captured on screen. Screenplay Formatting Breaking Down the Elements Celtx Blog from blog.celtx.com Type the following lines into it: The master scene heading is unique from other slug lines, in that it has its own structure when it comes to format. A straight perpendicular line on a film script means that coverage for the action it crosses has been shot.