Prepare a list of core topics and questions that you want your interview to cover. These should serve as your rough outline for the interview. Know that you’ll be asking lots of follow-up questions.
You’ll generally want to be looking for these things from the interviewee (all pertaining to your field of interest):
Their workflow. This will be the meat of the interview. You’ll be asking about the way they currently reach the goals you are targeting, who all is involved in that process, what tools and objects they work with, and what inefficiencies or problems they run into. You’ll also want to know when and how often they do a certain task, which objects are involved in the task and how many (generally and at most).
Their goals — why they are doing what they are doing.
Their mental model — how they perceive things, how they label and mentally group objects, how they understand the relationships between objects. You can often deduce a person’s mental model based on the words they use in descriptions of their workflow. You generally don’t need to prepare questions about the mental model up front, as they’ll arise from answers to other questions. Note that a person’s mental model may not always reflect objective reality (e.g. confusing “Android” with “Samsung”) — don’t correct them on that, you want to know how they perceive things.
You’ll want to prepare an overview question to cover the basics and encourage follow-up questions. For example, “describe the last time you wrote sheet music” would work if you’re working on a scorewriting application.