When people think about the challenges of interactive storytelling, this is the problem they think of most often: how to provide a consistent, well-formed story experience to the player if the player has great freedom of action? If you give the player a lot of freedom, he will have the power to subvert your story. There seems to be an inverse relationship between player freedom and story coherency.
There are three ways a player can destroy a game's story: violate the game world (by introducing things that don't belong there, often through speech); violate their own character (by acting in ways that are inconsistent with the way the avatar is defined -- in the lecture I used the example of Superman ignoring a baby crying in a burning building); and violating the plot itself, by doing something that results in an absurdity (e.g. destroying an object that reappears later in a predefined plot).
Of course, we usually prevent the player from doing these things by curtailing his freedom in various ways. Many games only offer pre-written lines of dialog, so the player can't talk about things that aren't part of the game world. Plots are designed so that the player cannot harm something that he will need later. (In the Grand Theft Auto games, you can't hurt the people who give you your missions, or even find them.) And if Superman ignores the baby, it's just a loss condition: game over.
Some designers, notably Chris Crawford and Andrew Stern, find this solution unacceptable; they feel that it is imperative to let the player do what he wants, and an interactive story must adapt to it. A few even think that we should never put a story in a video game at all, an absolutist point of view that I find ludicrous given their popularity.
The more I thought about it, the more I realized that we have constrained ourselves with some faulty and usually unstated assumptions about what interactive storytelling should be like. Here are the assumptions that we have made for years:
Once you abandon these assumptions -- which, as I said, are deeply rooted but largely unstated -- then interesting things start to happen. Over the years, various commentators have asserted that the designer and the player collaborate to create the player's experience, but they've seldom explained what this really means. In 2006 I suddenly realized that this collaboration arises out of the player's role in the game, and particularly how that role is defined. Role-playing is the fulcrum of the balance between story consistency and player freedom. Not role-playing in the CRPG sense -- leveling up and buying weapons -- but role-playing in the dramatic sense.
When a player starts to play a game in which he enacts an avatar character, he agrees to play the role that goes with the character. It's up to the designer to decide how strictly that role is defined. Superman, for example, has no moral freedom at all; a player who wishes to enact Superman signs up to Superman's moral constraints.
MMORPGs, on the other hand, frequently allow the player to build the avatar character from the ground up, and to enact a very wide variety of roles -- one of which might be to just hang around the entrance and give advice to newbies, if that's what they want to do.
However the role is defined, when the player chooses to play, he enters into a contract with the designer. The contract is very simple, and these are its terms: The designer promises to provide a credible, coherent story if and only if the player promises to behave in credible, coherent ways.
To put it another way, with freedom comes responsibility. The more power the player has within the game world to contribute actions to a story, the more responsibility he must take for those actions.
This abandons the assumption that the player shouldn't have to think about any rules; he has to think about the rules of drama. It also abandons the assumption that the designer is responsible for everything. If the player intentionally does something to screw up the game's story, then he, not the designer, is the one at fault.
(If he screws up the story unintentionally, that's a different issue. That's the designer's fault. It's up to the designer to construct a world in which the player cannot accidentally violate the story.)
In summary, I concluded that we still have the eternal trade-off between player freedom and story consistency, but it isn't such a big problem after all. It only seemed like a big problem because we assumed that we designers had total responsibility for anything that happened; that the player should be allowed to do whatever he liked and the story should respond appropriately; and that we were required to offer him maximum freedom at all times. By making those assumptions, we set ourselves up to fail. Instead, we need to embrace the trade-off and learn to work with it. The more power that we designers keep for ourselves, the more we are responsible for the quality of the story; the more power that we share with the player, the more the player shares in our responsibility.