When was the last time you thought someone was lying to you? Was it what they said, how they said it, or something else?

For Michele Rigby Assad, there was a time when determining whether someone was telling the truth meant the difference between life and death.

Assad spent a decade as an undercover CIA officer evaluating sources across the Middle East and an additional 14 years as a corporate and security advisor to companies operating in the Near East.

She says an indicator that someone may be lying isn't always what they say — it's when their behavior changes.

"We tend to see clusters of behavioral indicators that someone is not being forthright," she said.

Assad, who joined the CIA shortly after 9/11 and later specialized in counterterrorism, said she learned early in her career that many sources fabricated or embellished information. Spotting those lies became one of the most important parts of her job, she added.

Her advice isn't to look for a single universal "tell," such as avoiding eye contact or fidgeting.

Instead, she says the key is establishing what someone's normal behavior looks like before asking the difficult questions.

The biggest clue can be a change in behavior

Assad recalled one interview with an Arab intelligence officer she had been asked to evaluate for potential counterintelligence concerns.

After reviewing his case file, she said she developed a theory that he might be secretly working with another country's intelligence service while also providing information to the CIA.

She spent hours interviewing him, first establishing what his normal body language looked like before moving into more sensitive territory.

Rather than accusing him directly, Assad said she started naming countries she believed he could be working with.

"Then I got to the potentially problematic country," she said.

The man's reaction was immediate.

"He sat up, took the pillow from the couch that he'd been sitting on, pulled it out from behind him, and then balled up and started hugging the pillow," Assad recalled.

When she changed the subject, he relaxed and put the pillow back. A few minutes later, she brought up the same country. Once again, he grabbed the pillow. "He doesn't even know he's doing it," she said.

Years later, she said, she learned her assessment had been correct: the officer had been secretly working with another country's intelligence service.

Those kinds of behavioral shifts are more meaningful than any single gesture because they're deviations from a person's baseline, she said.

She saw similar reactions throughout her career.

One source built a wall out of Coke cans — a literal physical barrier between her and them — while answering uncomfortable questions. Another suddenly jumped out of his chair and ran to the bathroom after she asked about his wife.

The point isn't that hugging a pillow or leaving the room means someone is lying. It's that people often display stress in unexpected ways when they encounter a topic they'd rather avoid, she said.

Pay attention to what's missing

Assad said her biggest breakthrough as an intelligence officer came when she stopped focusing only on what people were saying and started paying equal attention to what they weren't saying.

In one case, she said she spent eight hours interviewing a source who was suspected of being a double agent. She finally left, convinced she'd learned nothing.

Nearly two weeks later, after reviewing her notes, she realized, "It wasn't what he said. It's what he didn't say that whole time I spent with him. That's the clue to this case," she said.

The source had never once spoken poorly of the government he was pretending to work against, but was actually secretly working for, she said.

For Assad, those subtle changes in behavior, combined with careful questioning and patience, proved far more reliable than the dramatic lie detectors often portrayed in movies.