Computer-based highly interactive drama involves different authoring approaches, compared to linear media. Underlying design principles needs to be understood in order to guide the authoring process, to teach authors and to design better systems. This paper identifies a fundamental design principle termed Structural Writing that underlies some of the most generative approaches in interactive drama. A theoretical description of this principle is proposed, which leads to a general architecture for interactive drama that may help authors and researchers to design systems that better exploit the principle of structural writing.