This paper investigates the dynamics of willingness to communicate (WTC) in written communication between a native and non-native speaker of English. Although research into WTC has identified topic as an interacting variable affecting L2 learners’ WTC during task performances (MacIntyre & Legatto, 2011) and classroom interactions (Cao, 2013), fewer studies have explored the nature of written WTC, its relationships with topics, as well as L2 writing development. This study addresses this gap by exploring the dynamism of WTC in writing from a Complex Dynamic Systems Theory (CDST) perspective. Through the employment of three methods—WTC analysis, complexity and accuracy analysis, and functional analysis—the findings suggest that WTC in written communication also reflects the characteristics of a dynamic system. Specifically, it fluctuates as the interlocutors organically move from one topic to the next. Results from the complexity and accuracy analysis and functional analysis provide further evidence to confirm that L2 development is a highly variable and nonlinear process. Overall, these findings lend support to the CDST perspective of interlanguage (IL) development, including dynamism in WTC and L2 writing development.