This introductory chapter theorises theatrimusicality as the mutually constitutive interplay of theatricality and musicality, proposing it both as a dramaturgical method and a hermeneutic frame for performances that inhabit the ‘in-between’ of theatre and music. It introduces spectaurality as the interpretive attitude that asks spectators to ‘see’ music(al) structure and to ‘listen’ to theatrical event, thus recasting perception as a chiasmic relationship between ocular and aural attention. Analytically, theatrimusicality troubles disciplinary binaries by foregrounding how gesture, movement and mise-en-scène shape musical meaning, just as rhythm, timbre and sonicity organise dramaturgical and scenic thinking; methodologically, the book seeks to bridge persistent gaps between musicological score-centrism and theatre’s ocularcentrism. Across case studies that range from avant-garde music theatre to sonic storytelling and choreomusical experiments, the introduction foregrounds how subsequent chapters underscore the ways in which spectauralising performance emancipates the spectator by empowering them as active co-makers of meaning within intermedial, relational spaces. The introduction maps this terrain, defines key terms and situates the book in dialogue with scholarship on musicality in theatre and theatre aurality, while arguing for a decentring of Euro-American canons through an emphasis on Asian and translocal practices.