Yes, your plan make sense, and it is common for the interview guides in qualitative research to evolve over time, as you both uncover new topics and become "saturated" with regard to other topics.

A different possible alternative is to pursue the same logic with a funnel-style interview guide. In that format, you begin the interview with broad, open-ended questions, and then in the middle, follow-up with increasingly specific questions.

@David L Morgan. I would prefer the first option, as the first series of interviews will be necessary to establish the more specific questions in the second series of interviews. Is there any specific term to search for to find this style of interview design? Or do you have any recommendations on authors?

Thanks for you help!

Grounded Theory is one approach to qualitative interviewing that consistently recommends an evolving interview guide. Good source in this regard would be Charmaz, Constructivist Grounded Theory.

Thanks a lot!

Julian Ganter - As a part of methodology you should mention the first as well as the second series of interviews in your thesis. Both unstructured and semi-structured interviews will help you explain, better understand, and explore research subjects' opinions, behavior, experiences, phenomenon, etc. By asking more open-ended questions it will be allowing for a discussion with the interviewee rather than a straightforward question and answer format. In this context, conducting the two series of interviews within one thesis paper make a sense.

Just remembered that I recommend exactly this design for focus groups, on pp. 45-47 of my book Basic and Advanced Focus Groups.

Dear Julian Ganter,

In my perspective the idea having two series interviews in the principles of unstructured and semi-structured interview is really possible. However, if you consider of using the semi structured interview in the process you can already apply the unstructured interview principle because there might be responses from the participants which requires follow up questions that is not in your semi-structured interview guide, that can be taken already as your unstructured interview. Moreover, unstructured interview is quite good since you can saturate what you wanted to explore from the participants and base on from their responses you can create new interview questions in response to the participants responses in the interview. But using unstructured interview requires skills since you cannot predict what may come out in the responses of your participants and what questions are you going to thrown out that correspond to your participants responses. If I may advise, you can utilize semi-structured interview so that you will be guided with the possible questions you would like to ask to your participants to capture their experiences in a certain phenomenon. Then if you would like to validate it and have more comprehensive generation of your themes you can have a follow up interview, but this time using the Focus Group Discussion or FGD interview. Julian Ganter