Customer interviews are built to make better decisions about your hypothesis and go straight to the source of the data.
Customer interviews are a sub process of customer discovery. If you reached this part it means that you found some potential customers to talk to and you need a good framework to get the data you need to go to the next step.
Customer interviews focus on the problem. Remember, you're not selling anything. You might not have anything to sell yet, so focus on the problem first.
It's crucial to understand to whom you need to talk to. Take your time to define the customer and remember that it's ok to be wrong, as long as you take a step back and reflect.
You need to be flexible during interviews. While you focus on the topic, it's important that you keep an open mind when new information arises. This way you can keep the interview going and collect more relevant and meaningful information.
If you remember that the goal is not to sell anything but focus on the problem, you will find out that you need to listen 95% of the time (and write everything down).
One last mention, you should strive for face to face interviews, and just as a fallback you should do phone and video calls.
During these interviews, you should find out who has the problem and how they deal with it. Your questions should be designed to gather quantitative and relevant data and you should avoid questions that lead to subjective and speculative answers.
To uncover important information about how the customer thinks, tailor questions to find more about the customers' habits. Also, avoid yes or no questions.
At the end of the interview, a good way to capture the next person to talk to is to ask: "Who else do you suggest that I interview?".
After enough interviews, you will start to see patterns emerge and information will start to repeat. This is a sign that you should end this phase, draw conclusions and go to customer validation phase. If you don't get here after 20-30 interviews, you are not talking to the right people.