Underpinned by MacIntyre et al.’s (1998) heuristic model, this study adopted an idiodynamic method to investigate fluctuations in the level of willingness to communicate in a second language (L2 WTC) in an online class. Seven EFL university students participated in four sessions of an online class (each lasting 20 minutes). Following the completion of each session, they rated their L2 WTC by filling in an online editable excel spreadsheet with a scale of -5 (not willing to communicate at all) to 5 (most willing to communicate) on a minute basis while watching a video recording of their performance. Subsequently, stimulated recalls and semi-structured interviews were combined to identify factors affecting moment-to-moment changes in their L2 WTC. Results showed that L2 WTC highly fluctuated during sessions 1 and 2 due to joint influences of trait-like (e.g., introverted) and state-like factors (e.g., technical issues). In contrast, a more stable pattern of L2 WTC was observed during sessions 3 and 4, mainly due to state-like factors, such as adequate support from a teacher. These findings suggest that EFL learners can become more willing to communicate in an online class through teachers’ affective, technical, and pedagogical support. Methodologically, this study shows that an idiodynamic method is a useful analytical approach by which to understand the fluid and dynamic nature of L2 WTC in an online classroom—an emerging L2 learning environment. Pedagogically, we offer insights into how language teachers need to respond dynamically to such factors during the course of an online class. We recommend that teachers choose interesting and familiar topics, integrate game activities into L2 tasks, provide learning stimuli (e.g., photos), upload recorded online lessons into their respective learning management systems, provide affective support by recognizing students’ efforts verbally or by using emoticons (e.g., a thumbs-up emoji) to make students feel connected with a teacher and other classmates, offer considerable wait-time for their responses during an online lesson, and promote peer encouragement and nurture a positive online learning environment.