The landscape of self-directed learning in mobile-assisted language learning (MALL) is changing from mechanical and repetitive to communicative and meaningful learning. This is due to the development and integration of conversational agents into mobile devices. Students can learn a language via a self-directed interactive app (SIA) where they can experience oral and textual two-way interaction with a conversational agent through their mobile devices anywhere and at any time using the target language. Despite this change, we know very little about students' actual use of this type of app in an informal setting or their level of motivation to use these apps. This study investigated students' usage patterns of an SIA in an informal setting and how the app supported students' basic psychological needs (BPNs) by drawing on the lens of self-determination theory (SDT). One hundred and seventy-nine Korean EFL primary students were introduced to an SIA, and their usage was tracked for eight weeks. An analysis of the data identified students' continuous or discontinuous app usage patterns, and a following SDT survey indicated that continuous students reported a more enhanced satisfaction of BPNs than discontinuous students. Additionally, findings from in-depth interviews provided further insight into how elements of the app supported or thwarted their BPNs. This study sheds light on the potential of SIAs as valuable tools for L2 learning in an informal setting. Future recommendations regarding using or developing SIAs for language learning were presented from both pedagogical and technological perspectives.