A friend who's a primary-care physician sent me a clinical interview guide last year. I started using three of its questions in customer research and they've quietly outperformed anything I picked up in a product course.
1. "Walk me through the last time this happened."
Specific. Past tense. Not what they'd do in general — what they did, in the last specific instance. Generalities are the worst possible source of product signal because everyone optimizes their own behavior in retrospect. The last specific time has texture.
2. "What were you doing right before that?"
This is the question that produces the actual problem statement. Customers are rarely solving the thing they think they're solving. They're solving the thing that happened ten minutes before.
3. "What would have to be true for you to not have called?"
Or, in software terms: what would have to be true for you to not have signed up? This is the inversion that finds the threshold. It also flushes out the substitutes — the spreadsheets, the Slack workarounds, the friend they texted instead.
Three questions, asked in order, with silence after each. Most of the value of a customer interview is the silence, not the words.