Empathy is going behind the expression and being able to tell degree and truth of the expression. I can smile at you at the same time something is bothering me elsewhere in my life or I can look "thoughtful" and if you can only tell happy/sad, that will generally look sad which may not be the case at all.

Empathy is having had a similar feeling about a similar event the person is discussing and not "feeling sorry for", but just "feeling for" the other's difficulty, pain, or struggle. It's not taking on the other person's feelings or experience as one's own but recognizing I am like the other person, have emotionally "been there, felt that". It is being able to sit with the other person while they share their difficulties and not be overwhelmed by them or distracted by one's own.