This thesis explores nonprofessional performances in fiction film. First-time untrained actors (nonprofessional actors) have been a recurrent feature in cinema since its inception and their presence has been frequently linked with modes and styles of filmmaking such as realism and "independent cinema". Despite their work in important films, nonprofessional actors and their performances have not received sustained critical attention. This thesis proposes to begin correcting this lack of scrutiny by studying nonprofessional performances, which it proposes to define as performances featuring details that reveal or suggest that the performer is not a professional actor. The primary aims of this thesis are to examine why and how exceptional nonprofessional performances contribute to the style and meaning of individual films. The first section of the thesis (Historical Perspectives) analyses how nonprofessional performance has been theorized and deployed in widely influential and historically significant film movements (Early Soviet Cinema in Chapter 1 and Italian Neorealism in Chapter 2). The aim of this section is to identify and explore different qualities associated with nonprofessional performances in films from the two periods. The second section of the thesis (Contemporary Approaches) offers close analysis on the on-screen activity of nonprofessional actors in three contemporary films. This section seeks to illuminate how specific films deploy nonprofessional performances to thematise questions regarding the ontology of film performance such as: how observation alters performance (Chapter 3), how repetition and exposure condition performance (Chapter 4) and how physiognomies affect the meaning of performance (Chapter 5).