Informal language learners from extended language immersion settings such as missionaries often have very disparate outcomes. Some learners achieve high proficiency levels while others fossilize in their interlanguage once they reach basic communicative competence. Could these differences in language outcomes be explained by learning strategies, motivation, or other psychological factors? To investigate these differences, 20 participants were selected based on participation in a post-mission language ability self-assessment resource (LASER) who had taken an Oral Proficiency Interview – Computer (OPIc). These learners were interviewed to probe beyond the LASER and discover what factors seemed to correlate with higher language proficiency outcomes. A qualitative analysis of these interviews revealed different factors that individually contribute to the success of individual learners. Some of these factors are interpreted as being consequences of proficiency rather than causes of it. Deliberate memorization of vocabulary and a sense of pride or perfectionism were hallmarks of higher achieving learners. Emerging across learners as key to higher proficiency was time spent in an immersive environment and with native speakers. This study has implications for autonomous or informal learners in identifying those strategies and motivations that have yielded success for others, as well as for the academic fields that study motivational, strategic, and individual variables in second language learning.