This article presents a reflection on what the qualitative interview method conducted with students can provide to (mathematics) education research in terms of in-depth knowledge and what critical methodological points should be taken into consideration. Repeated interviews with the same students in relation to research quality is considered. The argument is that repeated interviews can provide in-depth knowledge and a grasp of students' understandings. Critical points to consider when gaining in-depth knowledge are person-dependency, process ethics, connections between repeated interviews as a method and the aim, and the re-interview effect. These are important to discuss and reflect on throughout the research process, as they can function as quality criteria when producing in-depth knowledge in qualitative research with repeated interviews.