Excerpt from Telling Lies A failure to think ahead, plan fully, and rehearse the false line is only one of the reasons why mistakes that furnish clues to deceit are made when lying.
Mistakes are also made because of difficulty in concealing or falsely portraying emotion. Not every lie involves emotions, but those that do cause special problems for the liar.
An attempt to hide a feeling or conceal an emotion at the moment it is felt could be betrayed in words, but except for a slip of the tongue, it usually isn’t.
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