There is no magical formula as to how a perfect interviewer should look like. Two quite different kinds of persons can both be excellent interviewers. This does not mean that anyone can become a good interviewer. Certain things are a must-have for a good interviewer, e.g. proper education, specific characteristics, a respectful attitude, people skills, enough experience and expertise. The mixture may vary, but all ingredients are needed.
You do not have to be a master of every industry and profession, but you must have the general knowledge and also the intellect to understand what the people you are interviewing are talking about. How else can you interview them? The higher the position in question, the more demanding the task of the interviewer becomes. Interviewing a Candidate for a Sales Manager job in a 10 million turnover company is different from interviewing someone for a CEO job in a 1 billion € company.
Good communication skills and good listening skills are essential. A genuine interest in the Candidate and what is discussed in the interview is vital. The Candidate immediately senses if this is an interview of no real interest to the interviewer. A good interviewer is always well prepared and takes the time needed. Everyone understands that the interviewer may ask some tough questions, but this should always be done in a friendly atmosphere.
Experience, both work and life experience are a must-have. Having met and having dealt with many people gives perspective and understanding. And even so, you must continuously keep up with what is happening in the business world. Nobody is born ready into any profession. Neither does interviewing ten or even fifty persons make anyone a master. When you have interviewed 500 persons, you may have got a hunch of what this is all about.
You must also like people if you want to develop into a good judge of human nature. It does not hurt to have some intuition, instinct, gut feeling. Even though it is difficult to precisely define what “gut feeling” is, I rather have this than not. I feel this is your subconsciousness processing what you are hearing and seeing. Certainly, a good thing to have.
Then, no matter how experienced or knowledgeable we are, sometimes our personal feelings, bias, preconceptions, and prejudices may impact on our opinions and actions. It is not necessarily a conscious action. We cannot help reacting to the things we see and hear, so we must therefore consciously pay attention not to let this affect our professional opinions and actions in the wrong way.
There is probably much more required of a top-level interviewer, but this is for starters.