Composers of classic works of musical theatre carried with them tacit knowledge of the function of melody and harmony in relation to lyrics and dramatic action. This knowledge was underpinned by the cultural dominance of the piano and orchestra throughout the nineteenth and early twentieth centuries. With the rise of the guitar in popular music, very different musical forms began to challenge the scope of musical expression available to the theatre-composer-as-dramatist. This paper considers the kinaesthetic practicality of composition at the keyboard in contrast to composition on the guitar. It examines excerpts from a selection of musical theatre songs and points to melodic and harmonic resources deployed by the composer in support of the lyric and its dramatic intention. It poses further questions about the impact of instrumentation on the expressive range of music making for the future of musical theatre.