In many contexts of multilingualism, language learners can initiate communication in the target language (TL), or a contact language (such as English). Patterns of use emerging from these choices affect TL development. They also vary between individuals. Willingness to communicate (WTC) needs to be investigated in ways that capture these variations. So far, WTC has not been studied in multilingual contexts, or using individual-level longitudinal designs. Employing a single-case, time-serial design and focused on a critical period of TL growth, this study explores WTC trajectories of adult learners of Swedish for whom the TL and English provide viable communication options in community interaction. Change point and moving window correlational analyses reveal the operation of mutually interacting influences that shape WTC and have system-level effects. With light shed on processes at the developmental timescale, findings are discussed in the context of language choice, co-evolution, and the trait--state dichotomy.