Interviews are one of the tools and techniques for the PMI process to collect requirements. Interviewing is also one of the data gathering and representation techniques for perform quantitative risk analysis.
Interviewing is a way to gather historical or expert information. It can also be used to supplement other techniques to collect requirements from sample customers or users. The interviewer's skill is important. For instance, it is critical not to ask leading questions when gathering requirements. Or when interviewing an expert, the interviewer should allow the expert to provide complete answers and prompt for clarification of terms, methods, etc.
Interviews are discussed in the PMBOK, 5th edition, section 220.127.116.11, and interviewing is discussed in section 18.104.22.168.