Stress and ADHD is a crazy combination. I remember some of my early interviews — I was so petrified everything flew from my mind, and I couldn’t answer the simplest question. What I learned:
1) Prepare, as the other posts stated. Go through lists of typical questions and think of answers. Think of three strengths and situations where you used these strengths. Feel free to throw some of the prepared answers to the interviewer even if you are not sure they fit. It’s better than being at a loss for words. Surprise has always been my enemy in terms of coherence.
2) Never interrupt the interviewer. Sometimes we attempt to answer questions before they are finished, then realize we did not in fact understand it. Maintaining eye contact and reducing fidgeting will help, but if this is one too many things to keep in mind, relax. Some nervousness is expected.
3) Joking… that is a complicated issue. I know ADHD people who joke by default when nervous, so saying “do not joke” may be equivalent to telling them “do not sweat”. So I don’t know if thinking in terms of “not joking” is helpful. I will say to listen to the interviewer and try to give a short and factual answer to his question, without elaborating. If he wants clarification he will ask. Do not explain, just state facts.
4) What helped me the most is to keep in mind that the interview is a two-way street. The interviewer wants to find out if we are a good fit for the company, and we want to find out if the company is a good fit for us. And by that I don’t mean salary. Ask about the company culture — what is the motto of the company and how it inspires its every day dealings, do they participate in off work projects (volunteering, marathons etc.) as a company, do they seek to have a presence in the community, how is overtime handled (a diplomatic way to ask if it is expected) etc. Ask whatever is important for you, not because you feel you have to ask questions. See how the interviewer reacts to minor questions — is it a place where everything is top down, or do employees have input? A big part of the nervousness is that we feel put on the spot, and that somebody else has all the power. But companies look for good people also, and somebody with a master’s degree is expected to have a certain amount of critical thinking and personal goals.