I’ve been thinking about writing for digital, mobile and other new media.

So I not only looked at external media, I also drew upon what might be considered an unusual source for inspiration: a book about screenwriting.

Why screenwriting? Because Blake Snyder, author of Save the Cat! The Last Book On Screenwriting That You’ll Ever Need, gives great advice on how to focus on the essence—of a movie or any message—of “what’s it about.”

Here’s Mr. Snyder’s take: “If what the movie is about isn’t clear from the poster and the title, what are you going to say to describe it? If you can’t tell me about the movie in one quick line, well, buddy, I’m on to something else. Until you have your pitch, and it grabs me, don’t bother with the story.”

“In Hollywood parlance it’s called a logline or a one-line,” Mr. Snyder writes. “A logline is like the cover of a book; a good one makes you want to open it, right now, to find out what’s inside.”

You may be writing tweets, not screenplays, but the idea of the logline (also known as a high concept or a key message) works just as well. You’ve only got 140 characters or the three seconds it takes to read an email subject line or the eight words a web headline should be. You’re got to make those words count, and convey the essence simply, succinctly and in a very targeted way.

I love Save the Cat! and love the way Mr. Snyder characterizes a good logline: “You must be able to see a whole movie in it. Like Proust’s Madeleine, a good logline, once said, blossoms in your brain. You see the movie, or at least the potential for it, and the mental images it creates offer the promise of more. One of my personal favorites is the producer David Permut’s pitch for Blind Date: ‘She’s the perfect woman—until she has a drink.’ I see it. I see a beautiful girl and a date gone bad and a guy who wants to save it because . . . she’s the one!”

Is your social media writing “blossoming in the brain?” If not, read the book.