IMO most important is to listen more than speaking. Ask open ended questions, and be careful in your follow-up questions: as founder, it's too easy to be biased and ask those follow-ups in a way that's gonna bias the interviewee's response towards what you wanna hear. Be super mindful of that. It's very likely that you'll get responses that you didn't want to hear, and that's fine. This will help you define your product and ideal customer more precisely.

Also, after doing way too many user interviews my current conclusion is that building > customer research. It's been said hundred times before, but: Probably the best thing to do is to build stuff that solves your own problem. Then, IMO you don't need to do too much user interviews. The feedback you'll get from real users, and ideally paid users, is probably 10x more valuable than any interview.

Also: those are just my opinions, might as well be all BS :shrug: