When conducting interviews with multilinguals, researchers make (often invisible) decisions about the interview language(s). Whilst the research design may require a particular approach in some cases, linguists generally recommend giving participants a choice or interviewing them in their first language. There are ethical and methodological reasons for considering this, such as the implications for self-expression -- including emotion communication -- and therefore data generation and analysis. This paper offers methodological reflections about planning and conducting a research interview in which the researcher and participant knowingly share two languages, shining a light on the process of building linguistic flexibility into a study. The case study is an interview conducted in French and English, which explored a bilingual client's language use in psychotherapy. The paper gives practical insights into offering a choice of language(s) and planning for the possibility of a multilingual interview (i.e. code-switching). It considers how to mitigate language insecurities before illustrating how the interview language(s) may be negotiated in interaction. I argue for researchers to set clear interview language policies which foreground inclusivity, and show in the process that interviews can become multilingual exchanges, in which both interlocutors experience linguistic freedom.