Keep your Cool!
Emotions are not your friend when they rule your interactions with your difficult person. You need to be black and white, focused on the facts, calm, cool and collected.
It is in your best interest to NOT respond nor react when you are being ruled by your emotions.
Take time out. Be sure to arrange a follow up with your difficult person when you can get some perspective, when you can be calm, focused and professional.
For my Canadian readers, our current political confusion has been ruled primarily by emotions. Need I say more?
Be professional, take time out, stay focused and keep your cool.