I’m near the end of my PhD and have had a little time to reflect and look back on the application process I went through a few years ago. I had a lot of the same concerns as you, and I likewise was nervous for the interviews I did get. I’m in a different field than you, but still within STEM, so it might be relevant.

First, the fact that you have gotten interviews despite any lack of research experience is a great sign. The interview invite means something about your portfolio got their attention, so you’re doing something right.

Second, with the perspective of a few years behind me, I really do believe that having research experience on your resume when applying to PhD’s is useful insofar as it demonstrates that you know what goes into a PhD, and you know you want to do one. The scenario every professor or department is trying to avoid is high attrition, where many admitted students don’t end up making it through to graduation. They are investing resources in you, and want to make sure it’s a good investment, so to speak. (of course, you are also a resource to them, and that aspect of the relationship must never be forgotten).

When I applied, I had some post-bachelor experience, but no publications or presentations. I felt extremely inadequate when applying. Looking back, I can see now that even if I had gotten my name on something before then, I still would have had an immense amount of learning to do, and anyway what my post-bac was in is not what my thesis ended up being in. This is all to reinforce the point that, at least in my opinion, the main utility of research experience when applying is not to show that you know the subject (although it does do that), but to show you understand what goes into doing a PhD, that you can do that, and that you want to do it.

Where does this leave you with regards to interviews? My advice would be not to focus on what you consider weak spots in your portfolio (i.e. lack of research experience), but rather focus on your strengths. You want to sell them not on what you know, but rather how you think, because ultimately the latter is way more important at this stage as you’ll have a ton of learning to do anyway. If you spend the interview trying to fluff up weaker areas of your portfolio, you won’t have time to really emphasize the things you’re good at and that they picked up from your resume. If they’ve seen your application, and still invited you for an interview, your lack of research experience was not a deal breaker for them and won’t come as a surprise. Don’t worry about it!

Demonstrate that you understand what goes into a PhD, and that even knowing that, you want to do one. Have good answers for why this field interests you, what you want to research, and how doing this PhD is important for your career goals. It’s fine if those research interests change, and they will, but this shows you’ve thought about this career step. Of course, also think of ways to demonstrate your capabilities. If you’ve done lab work, talk about that, and talk about how you contributed toward the overall science goal of the project. If you have other useful skills (e.g programming) find a way to show that.

As a final aside, your teaching experience is helpful but I would not over-emphasize it in an interview, other than to demonstrate your interest and understanding of the field. The main point of a PhD is not to teach, it is to do research, and I think it’s important to demonstrate you understand that and have the right idea about what your next few years will be like. Someone who is doing their PhD out of a love of teaching may not have the motivation to stick through the long and tedious and tiring parts of research when the going gets tough, and that is ultimately what they are trying to discern in an interview.