Pursed lips, darting eyes or cross legs and arms "in and of themselves don't mean anything," he said. "But collectively, it can give you some idea of deception or anger or stress going on," said Garrett. "The most important key is what is normal for that person."
But he warned, "Any time you take a snapshot of behavior, you get in trouble. You look for a pattern."
Though polygraphers focus on standard lie-detection tests, reading emotions is a gift, according to Garrett, who has worked in hostage situations.
"I can't make you a hostage negotiator," he said. "I can tell you what to say and not say and the basics and running a command post. But not your ability to understand what that person is saying and what to say back at the right time and to read people. That's just the way it it is."
Columbia University scientists have recently mapped the two brain systems that are primarily responsible for allowing humans to accurately predict the emotions of others. The study, "The Neural Bases of Empathic Accuracy," was published in the Proceedings of the National Academy of Sciences.
Experiencing empathy involves assessing the entire situation within its context, a complex brain process that varies among people and is actually missing in those suffering from autism spectrum disorders.
What a person says, their non-verbal clues and the context of a situation determine how humans understand each other's emotions, according to study author and psychologist Kevin Ochsner.
"The TV show 'Lie to Me' vastly oversells what can be done," he told ABCNews.com. "Any given behavioral gesture you get from a person is completely ambiguous taken by itself. It can only be understood in the context of what is being talked about, to whom they are talking."
Psychologist Paul Ekman, upon whom the show is based, has established a facial action coding system to connect facial expressions to emotion, but Ochsner said reading emotion is more complex.
"They act like you can take a twitch of an eyebrow or failure to use their hands as indicative of someone lying. At the minimum, something is wrong," he said.
A presidential pursed lip is just a "static image of a person," according to Ochsner. "On the campaign trail Obama would talk about Hillary Clinton in speeches on the campaign trail, saying she's a real warrior and wiping under his eye with his finger. It's provocative, but can we draw any conclusions?"
Observing the president's "base rate" or the percentage of times he shows a non-verbal tic might reveal more underlying emotion, he said.
In everyday life, expressions can be deceiving.
"The salesman in the car showroom doesn't smile because he is happy," said Ocher. "He is counting on making a good impression."
So, too, the student taking an exam may look angry. "He's concentrating by furrowing his brow, narrowing his eyes and turning up the corners of his mouth."
Putting all the signals together allows humans to discern emotion and express empathy, according to Ocher.
Meanwhile, a dose of empathy might have defused the confrontation between the usually mild-mannered Henry "Skip" Gates and the well-trained Sgt. Crowley.
To Crowley, Gates may have appeared "aggressive or angry," according to Ochsner. But the officer was unaware the professor had just returned from a 20-hour flight from China.
"To Gates, the police officer may not have seemed concerned about trying to make everyone safe," said Ochsner. "He was following a set of rules because he had been told there was a break-in."
"When you have that information," he said. "It changes all the non-verbal clues."