Human-Computer Interaction (HCI) addresses problems of interaction design: understanding user needs to inform design, delivering novel designs that meet user needs, and evaluating new and existing designs to determine their success in meeting user needs. Qualitative methods have an essential role to play in this enterprise, particularly in understanding user needs and behaviours and evaluating situated use of technology. Qualitative methods allow HCI researchers to ask questions where the answers are more complex and interesting than "true" or "false," and may also be unexpected. In this lecture, we draw on the analogy of making a documentary film to discuss important issues in qualitative HCI research: historically, films were presented as finished products, giving the viewer little insight into the production process; more recently, there has been a trend to go behind the scenes to expose some of the painstaking work that went into creating the final cut. Similarly, in qualitative research, the essential work behind the scenes is rarely discussed. There are many "how to" guides for particular methods, but few texts that start with the purpose of a study and then discuss the important details of how to select a suitable method, how to adapt it to fit the study context, or how to deal with unexpected challenges that arise. We address this gap by presenting a repertoire of qualitative techniques for understanding user needs, practices and experiences with technology for the purpose of informing design. We also discuss practical considerations such as tactics for recruiting participants and ways of getting started when faced with a pile of interview transcripts. Our particular focus is on semi-structured qualitative studies, which occupy a space between ethnography and surveystypically involving observations, interviews and similar methods for data gathering, and methods of analysis based on systematic coding of data. Just as a documentary team faces challenges thatoften go unreported when arranging expeditions or interviews and gathering and editing footage within time and budget constraints, so the qualitative research team faces challenges in obtaining ethical clearance, recruiting participants, analysing data, choosing how and what to report, etc. We present illustrative examples drawn from prior experience to bring to life the purpose, planning and practical considerations of doing qualitative studies for interaction design. We include takeaway checklists for planning, conducting, reporting and evaluating semi-structured qualitative studies.