This study reports on anxiety, enjoyment, and breakdown fluency of L2 English learners under the monologue and dialogue conditions using an idiodynamic approach. Eight Mandarin-speaking undergraduates from a top university in China voluntarily participated in the study, with half completing in a monologue condition and the other half in a dialogue condition. The results showed that both monologue and dialogue groups exhibited a significant positive correlation between anxiety and breakdown fluency, and a significant negative correlation between enjoyment and breakdown fluency. However, these correlations were more pronounced in the dialogue group. The study also revealed that the monologue group experienced significantly less anxiety (p < 0.05) and more enjoyment (p < 0.001) in L2 speaking than the dialogue group. Nevertheless, there was no significant difference in breakdown fluency between the two groups (p = 0.152). Last but not least, the study found that learners' emotional fluctuations could be subject to cognitive, task implementation, task design, and interpersonal factors.