Do this before you write customer interview questions
Begin with the end in mind. What do you want to be able to say with confidence to yourself and your colleagues after the research is complete?
The best research is the kind where you can confidently make statements like "This segment of my customer base values my product specifically for x reason because it is what saves them a day's worth of work every week."
While we all wish we could ask our customers everything, you will be need to be targeted in your goal given the finite time for conducting the interview and compiling the research, Avoid the sprawling "I want to learn about my customer" interview goals. It takes ages to compile the research after the fact and you won't be able to draw enough conclusions from it since your sample will be diluted.
Starting with the end in mind helps you narrow down your questions, visualize which customers you will need to talk to, and estimate how much time you will need to conduct the interviews. Patterns emerge very quickly when you have a narrow focus.
Examples of conclusion statements:
1. Ecommerce start ups with 100-300k monthly visitors churn after six months because they sign up for our product as a temporary fix for x problem before they find a product that addresses the problem and scales better.
2. Women in their late 20's and 30's refer their friends to our booking platform more than any other segment does because they like to attend these types of events in groups.
3. Our agile project management tool is overkill for startups with <10 developers because they don't need the overhead of the many features that are built for multi-team dependencies and strategic program management.