Having the Actor-Network-Theory (ANT) developed by Bruno Latour in mind, I explore the fecundity of an approach considering manuscripts as actors on the stage through three contrasted case studies from the medieval Latin tradition. Firstly I examine the metaphor of the open book in the Dies irae, a famous late medieval liturgical poem. The central section investigates how the authority of Pope Gregory I is embedded into liturgical books and how this authority is transferred to later periods, especially through Ekkehard IV of St Gall in his continuation of the Casus sancti Galli. Finally the most spectacular case of a manuscript acting in a ritual performance is that of the ninth-century Purten Gospels re-used as a miraculous therapeutic instrument.