The through-line of action (also called the spine) is one method used by actors and directors to give a performance direction. It is linked to the theme or central idea of the play. This idea of a textual spine is a useful editing tool for the writer, too. It helps the author create a vehicle for actors by enabling the writer to decide what is theatrically important and what is textually superfluous in the script. In establishing a spine, the writer therefore makes every aspect of the play purposeful in terms of performance.
What exactly is the through-line or spine? The spine is a key idea or purpose of the play expressed as an action (usually a verb) which links all elements of the script (style, character, dialogue, action) and gives a coherence to the piece. The through-line enables both actor and director to narrow down the possibilities of action into core playable coherent concepts, providing clear motivation and direction to the actors in the way they perform an action or speak a line. In Two in a Room (see the section on Dialogue) the through-line is “to survive”. For a fuller example see “Rats!”case study.