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Good interviews are conducted by good investigators. There are multiple sources for information, but people are generally your best source in an investigation. They know the important details even if some of them can only fill in smaller factual gaps.
Interviewing is, of course, the process of asking and answering questions. An interview is a dynamic process between two people with multiple variables at work. An investigation has one goal: learning the objective truth. Using workplace interviews to reach that goal depends on your efforts.