This paper describes a two-month longitudinal study in which five secondary education students learning English as a foreign language at school will be interviewed in order to analyse how their motivation varies during this period and what variables affect their motivational changes. The emphasis will thus be on motivation's dynamicity and temporal variation. The data gathering will be based on semi-structured interviews, as this data elicitation method allows the researcher to follow a set of questions, while also allowing deviation from this guide to pursue and delve into motivational topics that arise during the course of the interview. The participants will be interviewed at three different stages throughout the two months and asked about the reasons for the motivational fluctuations that occur. The objective will be to try to pinpoint how the people-factor components (student, teacher, parents, peers and language assistant) determine the evolution of students' motivation and how they interact. The analysis of the type of interconnection that takes place among diverse variables and how they affect motivational changes over time will help to shed light on students' motivation from a complex dynamic system perspective.