I saw one of those documentary miniseries about the human body the other day. They say that blushing is actually the limbic system -- that animal brain that controls our baser instincts -- sending a rush of blood to the face as a way of signalling arousal. It's depressingly unsubtle, like those baboons whose hindquarters swell into rainbow-colored balloons when it's time to mate. There's an evolutionary value to maximizing any attempt at mating, even when the rational brain sees no reason for the limbic system's heady optimism. When you embarrass someone, their limbic system responds by flooding the body with chemicals, preparing to mate, or fight, or whatever it needs to do to keep a finger in the gene pool.
A young woman giggle at a forty-year-old CPA trying to still pull off the hockey jersey and backwards baseball cap look. He blushes. Why? Because his lizard brain, which doesn't get the joke, senses a mating opportunity, and it's making the most of it.
The 40-year-old gets drunk and announces to the bar, "Damn, her tits are nicer than Jennifer Tilly's!" She blushes. That little lizard brain is flooding her brain with chemicals to maximize her evolutionary potential.
What does this have to do with screenwriting? Everything. It shows that there's conflict in just about every situation, in just about every human being, most of the time. Every situation, no matter how simple it seems, has numerous layers, and those layers often produce the kind of conflict that makes a scene real. Being flustered isn't really one condition. It's different drives at war with each other. It's the rational brain and the lizard brain at odds. It's wanting to tell off someone you can't tell off. It's wanting an ice cream sundae when you know your dieting boyfriend can't have one. Most of our lives are governed by forces and desires in conflict.
Good screenwriting reflects that. So often I read screenplays where, in the end, most of what the main character did was talk about what he's doing. Many writers try to capture that inner conflict with dialogue about the conflict. It never works. Nope. Never. The answer is always simpler. Let your characters speak to you. Let them be hungry. Let them be horny. Let them be you. You think you're not getting it down on the page, because it's not there in so many words. But it is there, right where an actor can find it.
The quote from the Godfather above is a great example. You could write pages of dialogue and never get the different, conflicting thoughts that happen in anybody's head. All Mario Puzo and Francis Ford Coppola had to do is remember that Clemenza wouldn't just leave a box of fresh cannolis in the car.
Most of my good writing comes from the days I shut up and stop telling the characters what to say. On a good day, I can watch them blush, or remember there's cannolis on the back seat, or make a fool of themselves with the girl who finds them appalling.