What term is used for an actor who must use a script during a scene?

The term that describes an actor who must use a script during a scene is "on the book." This expression indicates that the actor has not yet fully memorized their lines and therefore relies on the written script. Being "on the book" is a common occurrence in rehearsal settings or initial performances when actors are still becoming familiar with their roles.

In contrast, "off book" refers to actors who have memorized their lines and do not need the script. "Scripted" generally refers to the content of a performance being predetermined, rather than relating to an actor's use of the script. A "prompter" is someone who assists actors by providing lines or cues if they forget, rather than describing the state of the actor themselves regarding the script. Thus, "on the book" accurately captures the situation for an actor dependent on their script during a scene.