Have you tried faking anger or sadness? Did you know that those are among the hardest emotions to fake?
It is because of the conflicted nature of such emotions. For instance, the quivering lip is linked to sadness and this takes place due to two sets of muscles pulling your face in different directions, and a part of your brain is trying to open the floodgates while another tries to control them. Other negative emotions, such as fear and anger, similarly bring about tension between competing sets of muscle.
Body language also has a big contribution to emotional expression. Lastly, the physical indications of emotional distress (such as goosebumps or sweat) are pretty much impossible to summon.