An in-depth interview (IDI) is a one-on-one qualitative interview, typically sixty to one hundred and twenty minutes long, conducted by a trained researcher with a single participant. Its purpose is to map a single decision, behaviour, or experience in enough detail that the researcher can understand the texture and the sequence of it.
An in-depth interview is not a survey. There is no fixed questionnaire. The researcher works from a discussion guide — a list of topics with possible probes — but follows the conversation where it goes. The most useful moments in an IDI are often the ones the guide did not anticipate.
Compared to a focus group, an in-depth interview gives up breadth and gains depth. There is no group to react to, no dominant voice to flatten dissent, no performance for an audience. Participants are more willing to admit confusion, contradiction, and second thoughts. Because the conversation is one-on-one and the researcher has time, contradictions can be explored rather than papered over.
Compared to ethnography, an in-depth interview gives up context and gains focus. An IDI is conducted on video or in a quiet room, not in the participant’s kitchen, so the researcher loses the environmental data — the placement of objects, the people walking through, the small frictions that ethnography surfaces. In exchange, the researcher can spend the full ninety minutes on a single, narrow question.
The right use case for in-depth interviews is when the question is specific and language-driven. How do customers describe a category before they know your brand? What is the precise sequence of objections that come up when someone considers switching? What words do they reach for to explain a churn decision? These are answered well in an IDI and badly in either a survey or a group setting.
Example: Before writing the positioning for a new D2C protein brand, the team runs twelve in-depth interviews with target customers to understand how they describe their fitness routine, their existing protein use, and the role of taste versus nutrition in their decisions. The interview transcripts produce the exact vocabulary the brand should be using on its packaging — and a list of words the brand should be avoiding.
Related: Ethnography, Focus Group, Diary Study.