Previous research into willingness to communicate (WTC) in L2 has focused primarily on its trait dispositions that remain stable across contexts and its situated nature is under explored. Framed with an ecological perspective on second language learning, this multiple case study investigated the dynamic and situated nature of WTC in second language classrooms. Based on data collected through classroom observations, stimulated-recall interviews, and reflective journals, it was found that situational WTC in L2 classrooms emerged from the joint effects of individual characteristics including self-confidence, personality, emotion and perceived opportunity to communicate, classroom environmental conditions such as topic, task, interlocutor, teacher and group size, together with linguistic factors. The findings suggest that language teachers should be mindful of the interdependence of all these involved factors that create students' WTC in class. (Contains 2 tables.)