People can say anything with words — flattery, agreement, enthusiasm — while thinking something entirely different. Robert Greene: "People can say anything they want to with words... our body language does not lie." Words are voluntary and easily weaponized as a mask ("I love your ideas, Cody. You're brilliant." while thinking otherwise); body language, vocal tone, and involuntary behavior are extremely hard to fake, even for skilled actors, because they leak genuine feeling.
Concrete cues Greene names:
Greene frames this as a decoding problem: the observer's job is to read the gap between the polite verbal mask and the true non-verbal signal underneath — an everyday lie-detection skill, not a special-occasion one. He wrote a full chapter on non-verbal communication in The Laws of Human Nature.
This complements Every Action Is a Sign (mundane behavior as signal) and depends on Observation Over Self-Absorption (you must be watching to catch it); it feeds Trust Your Gut Instinct About People (the felt read these signals produce).
Charisma on Command's How To Read People Without Them Knowing supplies a concrete tell-by-tell inventory, organized around three targets: interest, comfort, and emotion.
Interest:
Comfort: crossed arms, downward gaze, fidgeting, and especially a rigid/stiff body reveal discomfort someone is too polite to state outright — the cue to ease off before a pitch, sales close, or escalating first-date gesture backfires.
Emotion (framed as the hardest to read, since it's the area people are most trained at hiding): three channels give it away —
See Every Action Is a Sign for the epistemology underlying why these are combined rather than read individually, and Trust Your Gut Instinct About People for the instinct-based counterpart to this deliberate-observation approach.
Robert Greene (drawing on The Laws of Human Nature) argues humans evolved communicating non-verbally for hundreds of thousands of years before spoken language existed (Greene puts words at roughly 35,000–40,000 years old), so the brain is wired for non-verbal fluency but the skill goes undeveloped because people are "so word oriented." Greene's law number one of non-verbal communication: continually practice "muting the words" — shut off what someone is saying, as if muting a television, and watch only their behavior.
The surprise approach (sneak-up test): walk up to someone from an angle they can't see and greet them suddenly ("hey"); their immediate, unguarded expression in that split second — before their habitual mask returns — reveals how they really feel about you.
Micro-expressions last about 1/150th of a second but are detectable with practice; they leak a true reaction (e.g. disdain) before a person's composed mask reasserts itself.
Feet and posture as unconscious tells: feet pointing away from the person you're speaking with signal they want to leave, regardless of what they're saying; overall posture reveals confidence level.
Practice exercise: sit in a cafe far enough from other tables that you can't hear the conversation, and practice reading purely non-verbal cues without any access to the words.
Milton Erickson case study: Erickson — later the creator of hypnotherapy and (in "bastardized" form) an inspiration for NLP — was paralyzed by polio at 19 and spent two years unable to move, speak, or read. Greene attributes Erickson's later near-psychic ability to read patients entirely to those two years of forced, words-free observation, during which he catalogued "20 different forms of yes" and "a hundred different forms of no." Freud is cited making the same underlying claim: people continually "ooze out" their secrets non-verbally and can be read like an open book by someone who has mastered the language. Greene extends this even to seduction, calling it fundamentally a non-verbal language (gifts, scent, eye contact) rather than a verbal one.
See Genuine Smile vs. Fake Smile Detection and Dead Eyes as a Narcissist Tell for specific applications of this discipline to vetting romantic interest and toxic-personality deception.