Musicals are, by nature, theatrical, meaning poetic, meaning having to move the audience’s imagination and create a suspension of disbelief, by which I mean there’s no fourth wall.

Sondheim is arguing that the musical is inherently a heightened, openly artificial form. Because characters sing and dance, the medium cannot pretend to be ordinary “real life”; it must instead operate poetically—through compression, metaphor, rhythm, and theatrical convention—to engage the audience’s imaginative participation. His reference to “suspension of disbelief” reframes it as an active agreement between stage and audience: the show works when it invites viewers to accept stylization rather than hide it behind naturalism. Saying “there’s no fourth wall” underscores that musicals often acknowledge performance and address the audience indirectly, relying on shared understanding that what matters is emotional and dramatic truth, not literal realism.