9 customer interviews mistakes that can derail your plans

The mistakes that ruin an interview are mostly not made in the interview. They are made when choosing who to call, and again a week later, when the notes get turned into a conclusion nobody can trace.

So this runs along the timeline of one round of conversations, from picking names to writing the summary. The ones in the room are the middle third, and they are the ones people worry about most.

What to take away

• The two most expensive mistakes sit at the ends: who you recruited, and what you did with the notes.
• An interview that could not have produced bad news was not an interview.
• Write up the same day. A conversation summarized a week later has already been edited toward what you wanted to hear.

Recruiting

Screening for availability instead of relevance. You call the people who answer. Availability correlates with willingness to help you and not at all with having the problem.

Explaining the problem in the invitation. By the time they arrive they know the right answer and have had time to prepare a helpful version of themselves. Invite them to talk about how they currently do something, not about your topic.

Paying in a way that recruits professionals. An incentive attracts people who take incentives. If you offer one, keep it small, keep the screening strict, and note in your log which conversations were paid. That note matters again later: if you ever quote the person publicly, the payment is a connection that has to be disclosed, which the FTC endorsement guidance sets out in detail.

Talking only to current users of the thing you want to replace. They already selected into a solution. The people who looked and did not buy are the more informative group, and they are much harder to find, which is why nobody talks to them.

Booking and opening

Scheduling ten in a week. You will run the last six with the conclusions from the first four already in your head, and you will hear confirmation. Space them so there is time to write up between.

Bringing two colleagues. Two people on your side turns a conversation into an audience, and the answers get shorter and more polite. One interviewer, one silent note-taker at most.

Recording without asking properly. Ask before the call, not as the recording starts, and say who will hear it and how long you will keep it. Someone who agreed under pressure will be careful for the rest of the conversation.

Opening with your credentials. A minute spent establishing why you are worth talking to sets up an expectation that you know things, and people do not contradict experts. Open with a question about last week.

In the conversation

Talking more than a quarter of the time. Easy to check if you record: measure it once, and the number will be higher than you think.

Filling the silence. The tidy answer comes first and the real one follows if you wait. Counting to four is unglamorous and it works, as the interview mechanics set out at more length.

Defending the idea. The moment you explain the misunderstanding, the interview has ended and a sales call has begun. Write the objection down and thank them.

Accepting a hypothetical. Every question about would, could or might returns a guess. Every question about last time returns an account. This is the highest-yield habit in the whole practice, and it is not a quirk of founders: the guidance on writing survey questions documents how far a question's shape decides the answer even when a professional wrote it.

Interviewing in a group. The first confident person sets the story and everyone else agrees with a variation of it. Groups are useful for collecting vocabulary and useless for finding out what individuals do.

Closing

Ending on enthusiasm. Warmth at the end of a call costs nothing, and it is what you will remember. Make it slightly expensive instead: ask for the spreadsheet they described, an introduction to the person who signs, or half an hour to watch them work.

Not asking who else. The next name is the cheapest recruiting you will ever do, and it is the one route that repeats without a budget.

Leaving the buyer unnamed. Ask who signed off the last comparable purchase before the call ends. It is a factual question about the past, and people answer it.

Writing up and reading back

Recording conclusions instead of evidence. Notes saying "frustrated with the current process" have done the interpreting and thrown away the material. Keep what they said, what you observed, and what you concluded in three separate columns.

Waiting a week. Memory does not decay evenly. It decays toward whatever hypothesis you are carrying, and the parts that contradicted you go first.

Quoting the memorable one. The vivid conversation dominates the summary. Read the boring ones deliberately before deciding anything, because the ordinary case is the market.

Counting agreement. Tallying how many people liked it produces a number with no unit. Count workarounds, existing spend, and who could authorize a purchase instead, as in the discovery counts.

The check that catches most of them

Before the round, write one sentence: what would I have to hear to be disappointed. After the round, ask whether anything anyone said could have been that sentence.

If the answer is no, the mistake is upstream of everything on this page. Either the sample could not disagree with you, or the questions did not allow it. Both are recoverable, and both are cheaper to fix now than after you have built to the conclusion. It is the same failure the readiness mistakes describe from a different angle.

Common questions

How many people should be in a round?

Enough that you stop being surprised, which depends on how varied they are. Run them in small batches with a write-up between, and stop when a batch changes nothing in your written problem statement. Problem discovery covers what that statement should contain.

Is it a mistake to interview someone twice?

No, and it is underused. A second conversation months later, about what they actually did in between, is closer to evidence than any first conversation can be.

What if they ask what I am building?

Tell them at the end. Say at the start that you would rather hear about their situation first and will happily explain afterwards. Almost nobody objects, and the answers before the reveal are worth keeping separately from the ones after.

Can I trust an interview with someone who already loves the idea?

Trust it for vocabulary and for understanding the problem. Do not trust it for demand. Enthusiasm is free, and the only way to price it is to ask for something small and real, then watch. Whether that becomes a purchase is settled by market validation, not by another conversation.