Pacing is an extremely important part of writing. Solid pacing - knowing when big beats come, where the audience needs a short break, where all the highs and lows come - is an incredibly difficult thing to get down. It can take years of writing and many, many drafts of many scripts. I know I don't always have great pacing in my stuff, especially in first and second (and third) drafts. Effective pacing, however, will not only keep your audience engaged throughout, it will disperse information when necessary and - one of the first crucial steps in the process from page to screen for any script - keep your readers engaged.
When I write, because I work on so many action scripts these days, I like to outline before I start writing actual pages. This outline is always flexible, and the events, dialogue, and information within each scene often change from outline to page. What tends to remain more or less constant, though, is the structure - an informational and dialogue heavy scene here, followed by some action, followed by more info, and then, finally, an even bigger action beat. That is to say, the pacing remains the same, even if the content of the actual scenes does not.
Pacing, though, also means something else to me. Rather, 'pace' does. I hit my writing stride, and I don't want to lose or lessen my pace. I'm churning out the pages, really getting into the meat of the story, and the last thing I want to do is encounter that dastardly League foe, Writer's Block. Sometimes, though, I know that I have Scene A just about completed, and I know exactly what Scene C will be (and sometimes even how long it will be), but I have no idea what Scene B is that connects them. Instead of losing a half hour or more - and, more importantly, the pace that I've been working at - trying to figure out what the bridge between them is, I skip ahead to the next scene.
