Why do people write fanfiction? How, if at all, does fanfiction differ from the source material on which it is based? In this paper, we use quantitative text analysis to address these questions by investigating linguistic differences and similarities between fan-produced texts and their original sources. We analyze fanfiction based on Lord of the Rings, Harry Potter, and Percy Jackson and the Olympians. Working with a corpus of around 250,000 texts containing both fanfiction and sources, we draw on Biber's Multidimensional Analysis biber1988variation, scoring each text along six dimensions of functional variation. Our results identify both global and community-based preferences in the form and function of fanfiction. Crucially, fan-produced texts are found not to diverge from their source material in statistically meaningful ways, suggesting that fans mimic the writing style of the original author. Nevertheless, fans as a whole prefer stories with less focus on narrative and greater emphasis on character interactions than the source text. Our analysis supports the notion proposed by qualitative studies that fanfiction is motivated both by admiration for and frustration with the canon.