Emotions are difficult to cover up, and they reveal quite a bit about what’s happening behind someone’s eyes. While they aren’t a perfect indicator, emotions will normally match what’s being said. If they don’t, there’s likely more to the story.
If someone gets defensive in a situation where they aren’t under attack it’s reasonable to wonder if there’s something they aren’t sharing. If someone tells you they’re excited about something, but their face doesn’t reflect it, they might be paying lip service.