This portfolio thesis makes the case that participating in group devised theatre and using voice work benefits self-efficacy in additional language acquisition for advanced adult learners. The main source of evidence is the 'Performing Languages' project (2015) which took place at the University of Mulhouse, France, with adult participants from diverse cultural and linguistic backgrounds. I also draw on my earlier practice undertaken during the development of the thesis, including an article chapter that discusses a pilot research project, 'Experiencing the Word' (2013). My practice-based research engaged group devised theatre and its attendant emancipatory and democratic ethos, to improve the participants' self-efficacy in using the target language, English. I argue that an egalitarian ethos engendered by group devised theatre can address relations of power and the dichotomies of student/teacher and native/non-native speakers which are counteractive to achieving strong self-efficacy for language learners. The approach of group devised theatre also encourages and accommodates the renegotiation of identity that is prevalent in language learners. The overall findings of the research show that the benefits of participation in group devised theatre include: increased self-efficacy in using the target language, English; development of communication and engagement skills; and better awareness of non-verbal communication, gesture and a developed understanding of the use of the voice. Descriptions of moments of practice ('vignettes') are included to illustrate particular points of the overall thesis and to provide the reader with a sense of the practice and research from a phenomenological perspective. Also examined are aspects of the creative and learning process that occurred outside of the ensemble devising process; these were not immediately apparent but, I claim, give a fuller picture of both the process of group devised theatre and additional language development that took place. Furthermore, this thesis maintains that the unfinished and messy nature of SLA should be recognised and embraced in additional language development and that group-devised theatre provides an exemplary methodology for doing so.