You would probably benefit from learning some basic interviewing skills.
The first and most important preparation tool is to do some research. Know as much as you can about the other person or topic and have that background in front of you. It is amazing what five minutes on Google can tell you. Or just looking over the previous information and writing the questions you still are unsure of the answer to. By doing a few minutes of background research, your subject will know you cared enough to spend the time to get to know them.
Now on to the harder part, the questions. As Ryan Knapp so aptly put it, “asking open-ended questions to help prompt discussion/longer answers.” Which is exactly what you want! A discussion that feels like a conversation and less like a one-sided information session. Good places to start are why and how. Not very many people can answer a why or how question with a yes or a no!
Don’t be afraid of the follow-up question. If you didn’t get the answer the first time, try rephrasing the question and asking it again in another way.
Also, don’t just be thinking about what the next question will be and tune out the answer. The answer may lead to a different follow-up question that you wouldn’t have thought to ask if you had not been listening.
Finally, relax. Unlike journalists, you probably aren’t investigating a story. You’re just trying to get the required information to make your project or business relationship better.