User Interviews and their cousins the Focus Groups share a few traits across their disciplines. The chief trait is the moderator’s role with the interviewees. This article expertly breaks down the tenants of this relationship via the UX-side. Interestingly, observation is the most successful indicator. The author notes that, “put more emphasis on observing what people actually do rather than on what they say.”
Observing participants in their own context lets them show you exactly what they normally do.