DO project calmness: move and speak slowly, quietly and confidently.
DO focus your attention on the other person to let him know you are interested in what he has to say.
DO maintain a relaxed yet attentive posture and position yourself at a right angle rather than directly in front of the other person.
DO accept criticism in a positive way. When a complaint might be true, use statements like, "You are probably right" or "It was my fault." If the criticism seems unwarranted, ask clarifying questions.
DO acknowledge the feelings of the other person. Indicate that you can see he is upset.
DON'T use styles of communication which generate hostility such as apathy, brush off, coldness, going strictly by the rules, or giving the run-around.
DON'T reject all of the person's demands from the start.
DON'T make sudden movements which can be seen as threatening. Notice the tone, volume and rate of your speech. Keep you speech low and slow.
DON'T challenge, threaten, or dare the person. Never belittle the person or make him feel foolish.
DON'T try to make the situation seem less serious than it is.
DON'T invade the person's personal space. Try to maintain a space of 3' to 6' between you and the other person.
If this situation is life threatening, get out immediately. If necessary defend yourself. Call 911 as soon as possible.
Source: Utah State University, https://dps.usu.edu