Willingness to communicate (WTC), which plays a crucial role in second or foreign language learning, has a dynamic and complex nature, fluctuating during communicative events. This study examines how the situational WTC of four Japanese speakers of English as a foreign language (EFL) emerges and fluctuates, focusing on the roles of language proficiency and affective and conditional factors. Low-intermediate and advanced speakers were engaged in an interactive, communicative activity with an interlocutor. All utterances were recorded, transcribed, and analysed and combined with the qualitative data from participants' self-ratings and a stimulated recall interview after the activity to assess fluctuations in WTC. The findings indicate that levels of WTC are influenced differently between low-intermediate and advanced speakers. Although the WTC of all speakers was negatively affected when they sensed a lack of English proficiency, low-intermediate speakers were affected by interest in the topic, influence of interlocutors on sense of security, and self-confidence, while opportunity to talk about oneself and one's opinions contributed to the WTC of advanced speakers. It is suggested that teaches of EFL should be considerate of these factors that can affect EFL learners' WTC.