OK, I’m fibbing, it was actually Tennessee Williams’ advice to playwrights, but it applies just as much to screenwriters:
“What shouldn’t you do if you’re a playwright? Don’t bore the audience! I mean, even if you have to resort to totally arbitrary killing onstage, or pointless gunfire, at least it’ll catch their attention and keep them awake. Just keep the thing going anyway you can.”
Of course you don’t really want totally arbitrary events in your script, but if you need to capture their attention, put it in and then in the next draft work your way backward in the story so it has some motivation or at least is foreshadowed and work your way forward in the story to make sure it has a consequence.
(Jurgen Wolff offers a new screenwriting tip here every Tuesday; also see his site, www.ScreenwritingSuccess.com and his book, “Your Writing Coach.”)