This study aligns with the recent calls in L2 Willingness to Communicate (WTC) research that mark a shift from viewing WTC as a static, trait-like variable to a construct that is dynamic, fluid, and situational. To accomplish this aim, this study collected both interactional and interview data to examine the situational changes of an ESL learner's WTC. The data was from two courses -- one ESL course, and one academic graduate seminar -- collected during the participant's first semester at an American university. The findings show that the participant's WTC fluctuates according to the course topic, interlocutors, group size, and the anxiety level of the L2 speaker. These findings imply that teachers in L2 classrooms need to take into consideration the various situational factors that promote or inhibit WTC. Other than these pedagogical implications, the study also offers a methodological framework for documenting situational WTC by means of combining interviews with conversation analysis of interactional data.