Emotions can be fleeting. At times faces change so fast that it is hard to figure out any one expression. It helps to understand that usually expressions stay within areas of the Emotion Color Wheel. That is, if someone is happy, he will usually stay in a circle or emotions around happy) unless something happens in the situation, like someone insults him.
Then he might jump across the wheel to work around an angry circle of emotions. People don’t usually jump quickly across the wheel without reason. If you are not sure of the expressions, you are usually OK if you stay in the outside of the Emotion Color Wheel in your responses and pay attention to changes in the situation that could make someone shift emotion strongly. Sudden shifts tend to be toward the center of the wheel, which can create tension.
Below are some videos where people change emotions. See if you can tell when a new emotion starts and how the faces transitions between emotions.
Content -> Confused -> Relieved:
Happy -> Unsure -> Distate -> Disgusted:
This video shows a range of emotions, from happy to interested to upset: