The role of communication variables such as communication apprehension (CA), self-perceived communication competence (SPCC), and intercultural willingness to communicate (IWTC) in improving L2 WTC has rarely been compared or even considered in detail in project-based learning (PBL) classroom contexts. Given this research gap, a mixed-methods study was designed to examine the interconnection of the above-mentioned communication variables in PBL contexts. A questionnaire was administered to a total of 82 senior-year students from four sections of a PBL course at a university in northern Taiwan at the beginning and the end of the 18-week semester. The findings showed that SPCC and IWTC had significant explanatory power for L2 WTC, with the former exerting a greater impact on L2 WTC. Furthermore, with the exception of CA, the results for all the communication variables indicated significant positive change towards the end of the course. To gain more insight into the cognitive, affective, and contextual factors that may influence L2 WTC in PBL contexts, qualitative data derived from reflection reports revealed that situational WTC in PBL L2 classrooms was regulated and mediated by the interplay of multiple factors: group member support, learner engagement, mastery experience and practice, intrinsic motivation, and positive feedback and interaction.