Quick answer: An in-depth interview (IDI) is a one-on-one, semi-structured conversation designed to uncover the why behind a customer's behavior — their motivations, mental models, and decision triggers — in a way surveys and analytics never can. You typically need 15–30 interviews to reach thematic saturation for a focused audience (and 20–40+ for a heterogeneous one). The catch has always been cost and time: a skilled human moderator can run only 8–10 IDIs per day, and doubling your sample roughly doubles your budget. In 2026, AI-moderated interviews break that ceiling — running hundreds of IDIs in parallel, 24/7, with automatic analysis — so you get interview-grade depth at survey-grade scale.
What is an in-depth interview?
An in-depth interview (also called an IDI or one-on-one interview) is a qualitative research method where a single participant is guided through an open-ended conversation about a specific topic. Unlike a survey, an IDI is adaptive: the interviewer listens, then probes. When a participant says "the checkout felt confusing," a good IDI doesn't move on — it asks "confusing how? Walk me through exactly where you got stuck."
IDIs sit at the depth end of the research spectrum:
- Surveys capture what at scale, but stop at the first answer.
- Focus groups surface group dynamics, but suffer from groupthink and dominant voices (see our focus group guide).
- In-depth interviews capture the individual why — the richest signal, one person at a time.
They're the workhorse of customer discovery, empathy interviews, jobs-to-be-done research, win/loss analysis, and value-proposition testing.
When to use in-depth interviews
Reach for IDIs when you need depth over breadth:
- Early discovery — you don't yet know the questions to put on a survey. IDIs help you find them. (Our customer discovery guide covers the founder use case.)
- Sensitive or complex topics — cancellations, pricing objections, workflow frustrations that people won't detail in a text box.
- Understanding a decision — why a buyer chose you (or a competitor), the story behind a churn event, the trigger that made someone finally look for a solution.
- Explaining the "what" — your analytics show a drop-off; IDIs tell you why it happens.
If you mostly need to quantify a known variable, a survey is cheaper. The strongest research programs combine both — see our mixed methods guide.
How many in-depth interviews do you need?
The honest answer: enough to reach saturation — the point where new interviews stop surfacing new themes. In practice:
- Homogeneous audience, focused question: 15–30 interviews is the classic range, and saturation often appears around 12–20.
- Heterogeneous audience or multiple segments: plan for 20–40+, because each distinct segment needs its own path to saturation.
A well-known caution from the qualitative literature: many published studies claim saturation they never actually reached, because the sample was too small or too narrow. The safeguard is to keep interviewing until two or three consecutive sessions add nothing new. For a deeper treatment of who to talk to, see qualitative research sampling methods.
How to run a great in-depth interview
1. Write a discussion guide, not a script. List 5–8 open-ended topics and let the conversation breathe. Use our user interview script template as a starting point.
2. Open broad, then narrow. Start with context ("tell me about the last time you…") before drilling into specifics. This grounds answers in real behavior rather than hypotheticals — the core lesson of The Mom Test.
3. Probe relentlessly. The insight is almost never in the first answer. Master probing questions and the laddering technique to move from features to underlying motivations.
4. Avoid leading questions. "Wouldn't it be great if we added X?" contaminates your data. Learn to spot and avoid leading questions.
5. Shut up and listen. Aim for the participant talking 80% of the time. Silence is a tool — it invites elaboration.
6. Capture verbatims. The quote is the deliverable. Record, transcribe, and tag every session so themes are traceable back to real words.
Common pitfalls — asking hypotheticals, pitching instead of listening, stopping at the first answer — are covered in 15 customer interview mistakes.
The scaling problem (and why 2026 changed it)
Here's the structural weakness of traditional IDIs: they don't scale linearly, they scale painfully. A skilled moderator can run perhaps 8–10 interviews a day. Twenty interviews means two full days of moderation, plus scheduling, no-shows, transcription, and a week of manual synthesis. Doubling your sample doubles the cost and time. That's why so many teams settle for five interviews and call it "research" — and then over-generalize from a sample that never reached saturation.
The data on the alternative is striking. AI-led interviews now deliver insights up to 100× faster and at roughly 75% lower cost than human-moderated sessions, while still capturing nuance. AI platforms can run hundreds of interviews overnight and return actionable insights within 24 hours. A popular hybrid pattern: run 200 AI-moderated interviews to map the landscape, then hand-pick 15–20 for human follow-up on the sharpest themes.
How Koji runs in-depth interviews at scale
Koji is an AI-native research platform built to run true IDIs — not glorified surveys. You write your discussion guide once, and Koji's AI moderator conducts voice or text interviews with every participant, probing each answer in real time exactly like a skilled human researcher — asking "compared to what?", "can you give a specific example?", and "why did that matter?" automatically, with no moderator bias, no scheduling, and no fatigue on interview #200.
Because Koji supports six structured question types — open_ended, scale, single_choice, multiple_choice, ranking, and yes_no — you can blend the depth of open-ended probing with quantified measures (a satisfaction scale, a ranked feature list) inside the same study. Then Koji does the week-long part instantly: automatic thematic analysis clusters every transcript into themes, sentiment scoring flags the strongest reactions, and a one-click report hands stakeholders the verbatim quotes plus the distribution behind them.
The result is the thing IDIs could never offer before: interview depth at survey scale, from question to insight in hours, not weeks — with no research expertise required. Whether you need unmoderated reach or moderated depth, you no longer have to choose (see moderated vs unmoderated research).
Run your first in-depth interview study
Draft a 5–8 topic discussion guide, then let an AI moderator conduct — and probe — every conversation for you. Launch your first AI in-depth interview study on Koji and reach saturation in days, not months.
