A depth interview, often called an in-depth interview or IDI, is a core qualitative research method in which a researcher engages a single participant in an extended, semi-structured conversation. The goal is to move beyond surface-level responses and reach the underlying beliefs, emotions, and decision-making processes that drive behavior. Depth interviews are particularly valuable in consumer and market insights work when researchers need to understand complex purchase journeys, test early-stage concepts, or explore sensitive topics where group dynamics might suppress honest responses. Because each session focuses on one participant, the method produces rich, contextually grounded data that surveys and focus groups rarely match.