This study used 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 took part in four online class sessions (each lasting 20 min). They rated their L2 WTC after each session while watching a video recording of their performance. Following that, stimulated recalls and semi-structured interviews were used to identify factors influencing moment-to-moment changes in their L2 WTC. The findings indicate that EFL learners' levels of L2 WTC are highly dynamic as a result of the combined influences of various trait-like (e.g. openness to a new online learning experience) and state-like factors (e.g. technical issues) factors during their participation in an online class. In practice, these findings indicate that L2 teachers can continue to play an important role in increasing students' active participation in an online classroom by creating a positive learning environment through affective, technical, and pedagogical supports. This study demonstrates, methodologically, that an idiodynamic method is a useful analytical approach for comprehending the fluid and dynamic nature of L2 WTC in an online classroom--an emerging L2 learning environment.