Possibilities: Not only do they add complexity, they add suspense. In a plot-driven story thread, we explore all the possible plot options. In a character-driven story thread, we explore all the possible character interactions.
Whether you do as an author, your audience will explore the possibilities. And as they explore all the options, they will sit on the edge of their seats longing to find out which one comes to pass. But I’ll get back to that in a moment.
As multiple story threads interact, the possibilities grow exponentially. Not only that, they spawn more story threads. Each event in a story may spawn or terminate a story thread, or join it to another thread. The main character causes some of these events. Others happen because of other characters or even as happenstance.
In an interactive story, your audience generates events. Usually these events are caused by the main character, but there’s no conceptual reason why they must be. The hard part in a complex interactive story is to follow all the story threads and to make sure they all end up in a satisfactory place. In any case, your audience will follow all these story threads, all these options. They’ll sit mesmerized watching the events play out. The fact that they themselves generated some of these events only enhances the experience. Give a person control, and they’ll become emotionally invested in the result. Besides, other events are out of their control, and the number of possibilities is still innumerable.