Since you say "carrying out an interview," I assume you are asking about the skills that are needed to be an effective interviewer in a business context.
If so, the greatest skill that is needed in interviewing is the ability to listen effectively. An interviewer must listen closely to the answers given by the interview subject. Interviewers must be able to find areas that need to be followed up. They must be able to have a sense of the body language of the person being interviewed. An interviewer who is able to listen effectively in these ways will be able to conduct more productive interviews that are more likely to truly determine how good a given candidate would be for the position in question.