Interviewing is not an easy task and not many people can do it either. In my opinion, a good interviewer is the one who reads through profile of the candidate, before taking an interview. An interview that starts slowly and makes the candidate comfortable can be termed as a good interview. Questions that must be asked at first should be basic, so that a conversation is started and the candidate does not feel nervous. Basic questions could be a brief history of his achievements, which can be later followed by the interviewer appreciating him and a general idea of members of his family. Questions should usually be open-ended which either makes the subject wonder what his visions are and if in his few year, would he be able to accomplish the same. An interviewer should not force the candidate to answer but provoke him to speak as much as he can.
The important thing that should be kept in mind here is that, that the interview is basically selling his company to the candidate as he is representing from the side of the company, thus he should make a good impression of himself and the company. It does not matter if you are hiring him or not, the candidate would always have a good point of view about the company and might recommend it to others too.
Another major thing which I believe shapes a good interview is when the interviewer is having fun. If the person who is interviewing makes an interview looks like a chore, then the interview can grow dull or awkward but if the person makes the interview fun, then a good rapport is formed between both, the candidate and the interviewer which helps develop new relations and generate amazing content as answers.