This article reads a series of essays on the actor by James Boswell through recent scholarship on the theory of acting in order to elaborate an expansive and historically grounded definition of what was and is meant by '18th-century acting theory'. I thus show how 18th-century texts on acting are important documents that should be read not as isolated phenomena but as works that can illuminate contemporary stage performance and the culture that produced it. In particular, I follow Boswell by placing a specific, illustrative emphasis on three key themes of professionalism, theatrical expression and ephemerality: each theme is both essential to thinking about the stage (and criticism on this topic) while also, like so much about the 18th-century theatre, applicable far more widely both then and now. K E Y W O R D S 18th-century, acting, Boswell, rhetoric, theatre 1 | INTRODUCTION One sometimes sees the study of 18th-century theatre divided into two complementary processes of extraction and reintegration. The former, by cutting 'the theatrical notices out of newspapers' and other such work, has allowed the construction of vital calendars and profiles, now given new life as online databases (Burkert, 2019; Roach & Robinson, 2020, p. 188). The latter has endeavoured to return such data to its original place and freshly observe how they functioned 'as significant elements within a wider social and cultural context' (Roach & Robinson, 2020, p. 188). This second way of thinking about 18th-century theatre has been particularly important for the This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.