Let's say you find yourself in the middle of a test session and the user you're with isn't doing the best job of articulating their thoughts and feelings. You should remain vigilant in listening to the participant, but you can also focus more of your attention on their nonverbal behaviors like posture, hand gestures, facial expressions, etc.
Key Lime Interactive's CEO, Ania Rodriguez, and User Experience Researcher, Matt Bruce, as demonstrate common situations in UX research and some of the more relevant emotions to pick up on while observing users.