This thesis investigates the engagement of prominent British literary writers in adaptations of their works to theatre and film between 1823 and 1938 to understand how unregulated adaptations, new media, media rivalries, the variable position of writers within and across media, and other social, cultural, economic, and legal contexts within the nineteenth and early twentieth century worked together to create power struggles, binaristic boundaries, and cultural prejudices that both promoted and limited adaptation across media. Using an interdisciplinary, historical, cultural, and analytical approach, this thesis traces continuities and changes between theatrical adaptation of prose fiction in the nineteenth century and film adaptation of both prose and plays in the early twentieth century, focusing particularly on how literary writers adapted themselves and their writing to shifting media contexts, both over time and across media within the same period. This thesis argues that dramatic adaptation practices in the nineteenth century, themselves shaped by cultural and socioeconomic contexts, shaped literary writers’ engagement with early film adaptation practices. Whilst rivalries between writers and media were fuelled by medium specificity theories and Romantic theories of originality spurned adaptation, this thesis finds that some literary writers challenged hierarchies of adaptation through presenting adaptations as originals (and vice versa), while others defied medium specificity through experimental, hybrid, cross-media writing whose dismissal precluded promising intermedial collaborations and aesthetic innovations in film. The historical analysis of polyvocal dialogues thus informs critical and theoretical debates on writing, authorship, and adaptation.