Interactive storytelling is defined as distinct from interactive fiction (or IF), as well as video games with strong narrative focus ("Mass Effect", "BioShock", etc.), by user agency and open-ended narrative. David Gaider, an RPG developer at BioWare, stated that "every possible branch needs to be written and fully realized, even if not every player sees it, and thus any game which allows for a lot of player choice becomes a much more expensive proposition for a developer." IF and video games, to balance user choice with authorial effort, must constrain the directions the narrative can take with puzzles, battles, or unchangeable plot points and bottlenecks, all of which detract from a sense of immersion. Only the most critical of the user's narrative choices are used or remembered in narrative development, according to the need to fulfill specific player goals that define a "gameplay" experience. A true IS system would incorporate all of them, as do living human agents, simultaneously and continuously - a task only artificial intelligence can meet. Sandbox games like "The Sims" and "Spore", which do involve extensive AI-based social interaction, do not manage dramatic tension or produce a cohesive narrative .