British Sign Language
During a BSL interpreted performance, every word that is spoken or sung is interpreted into British Sign Language (BSL) by an interpreter standing on stage, so that Deaf members of the audience can understand the play and follow the story.
Doing this well depends on an in-depth knowledge of the script, and the two languages. BSL is a visual-gestural language that is distinct from English, with its own grammar, structure and syntax.
In conveying the meaning of every line uttered on stage, they effectively ‘perform’ the play for the Deaf audience and can be thought of as an additional performer on stage.