This paper explores new authoring paradigms and computer-assisted authoring tools for free-form interactive narratives. We present a new design formalism, Interactive Behavior Trees (IBT's), which decouples the monitoring of user input, the narrative, and how the user may influence the story outcome. We introduce automation tools for IBT's, to help the author detect and automatically resolve inconsistencies in the authored narrative, or conflicting user interactions that may hinder story progression. We compare IBT's to traditional story graph representations and show that our formalism better scales with the number of story arcs, and the degree and granularity of user input. The authoring time is further reduced with the help of automation, and errors are completely avoided. Our approach enables content creators to easily author complex, branching narratives with multiple story arcs in a modular, extensible fashion while empowering players with the agency to freely interact with the characters in the story and the world they inhabit.