Emotions, our state of mind, our feelings, how we react to what is going on. Emotion can have many different definitions, and be seen differently by different people. In addition emotions can also be viewed on a dual level; what we feel internally, and how we express that externally.
One way of showing emotions externally is through facial expressions. Some emotions are easier for others to notice on a person's face such as pain and anger. When I'm writing, I like to try and visualize my character as they are expressing an emotion. I try to see their eyebrows lower in anger, and their eyes narrow. And I try to consider what reaction that will cause in the other characters they are interacting with.
What about your characters? How do different situations effect your character's emotions? How do they express those emotions? Are they more likely to talk out their pain or frustration with other characters? Do they use other characters as scapegoats? Do they try and pretend they are fine?
Yes, I guess I focused on more "negative" emotions here. I could have mentioned more "positive" ones. Though with fiction the ones that cause conflict between characters are more interesting.