Don’t scratch your ear — even if it really itches. If you have a thought you want to come back to, quickly jot down one memorable word, not an entire sentence. And remember: that camera in front of you represents 60 million sets of eyes.

Seasoned debate coaches bombard their candidates with tips like these, warning that even the slightest gesture or facial tic can make the difference between a performance won and a loss.

Their biggest fear? The dreaded split-screen shot, which has long bedeviled presidential candidates who — no matter how many times they are reminded — seem to sometimes forget that they are still on camera even when they are not speaking. Last week, President Obama became the latest to fail to put on what people in the television business call “neutral face,” a warm but flat expression that betrays no hint of whatever feelings might be simmering inside.

And thanks to the president’s lackluster debate, when he was often shown looking down or appearing otherwise disengaged as Mitt Romney, his Republican opponent, spoke, the split-screen should not be far from the minds of Vice President Joseph R. Biden Jr. and Representative Paul D. Ryan when they meet on Thursday night.