Science educators demonstrated that students' framings -- their expectations of what is going on -- influence how they participate, and what science knowledge they reveal, in clinical interviews. This paper complements research that explores how interviewers are likely to affect student framings, by exploring how subtler interactions can lead students to change their framings, and thus their behaviour, in unexpected ways. We present data from interviews with two students, Sarah and Omar, as they reasoned about evaporation and condensation. Sarah demonstrated spontaneous changes in how she participated over the course of the interview, whereas Omar demonstrated subtler changes that existing methods may not capture. These changes affected the nature of scientific knowledge and reasoning demonstrated by each participant, but could not be fully explained by response to interviewer behaviour. We use the constructs of footing and positioning theory to examine how students participated during the interviews, and how this affected the ways they demonstrated knowledge and reasoning about the interview topic. In both cases, footing and positioning theory allowed us to better understand the dynamic ways students engage in the interview. This paper contributes new methods for analysing complex interview dynamics, and suggests situations for which such methods are necessary.