The piano music of Robert Schumann has long prompted discussions about the relationship between the physical and the ideal, utterance and imagination. In the area of Brahms studies, scholars have analyzed how, in Brahms’s works for piano, allusions and counterpoint often gain affective meaning in the act of performance. This dissertation filters these aspects through the dual lenses of the innere Stimme—a form of internal vocalization encapsulated in Schumann’s Humoreske, Op. 20—and what I call “four- handedness”—the evocation of textures and gestures from four-hand piano playing in music for two hands. I argue that Schumann and Brahms used both as strategies to conjure the presence of an imaginary co-performer in select works for solo piano. Such moments simulate for the soloist notions, memories, and actual acts of collaborative music making that can unleash highly charged private experiences in light of biographical circumstances. Part I lays the groundwork by developing these two lenses and assembling a range of musical examples for each to illuminate broader trends in nineteenth-century piano writing. Part II culminates with case studies on Schumann’s Impromptus on a Romance by Clara Wieck, Op. 5, and Brahms’s Variations on a Theme by Robert Schumann, Op. 9, where circumstantial evidence points to the innere Stimme and four-handedness as fertile channels for experiencing and communicating musical knowledge and intimacy. Taken as a whole, the dissertation adds to our understanding of the stylistic history and performance practice of the more intimate forms of piano music, and contributes to a larger picture of how music operated in the private sphere to forge social relationships in German musical culture.