However, it is often difficult to make decisions without making use of this information; my character might decide to head north instead of south, merely because as a player I happen to know what the GM said about the north (or south) to another player.

It’s called “meta-game thinking”, and before you start saying “who cares”, it’s applicable to more than just Dungeons and Dragons.

Let’s take Law & Order, for example.  Each episode is generally self-contained; they find a suspect or three, investigate them, and take one of them to court.  Recently I watched an episode guest-starring Bob Saget.  He wasn’t the initial suspect; he wasn’t even the second or third suspect.  But the instant I saw him, I knew he would be the ending suspect – I used information outside of the show’s universe to extrapolate the outcome of the show.

Specifically, more famous guest-stars are always the final suspect.  Now, a viewer who was unfamiliar with Bob Saget would never have been able to determine the outcome so early; it was only my knowledge of the actor’s fame that allowed me to make that determination.

Similarly, even when actors aren’t as famous, you can guess things based on meta-world information.  For example, in V, Alan Tudyk plays a V who is undercover as an FBI agent; he dies in the third episode.  But a friend of mine was able to guess that fact, before it was revealed, because Alan Tudyk was credited as a “guest star”.  Guest stars are not main characters; using that meta-knowledge reveals things about the show that you would otherwise not know.

This is the biggest problem with TV shows.  Merely knowing who the actors are can be enough to reveal “secret” pieces of the plot.  I don’t know what to do about it, but it does sometimes ruin the immersion into a show’s plot.