I first started thinking about this with Brandyvein Beetles. They make dungeon explorers drunk which is a totally cool idea if it were in a piece of fiction or a movie. But the only way to really simulate it at the table is to push the responsibility onto the player: "Okay, pretend you're drunk" (or have players like mine, which are well on the way to "simulating" drunkenness no matter what the situation, hah).
There are a few effects that the system can handle decently. Because the DM controls access to knowledge of the game world, effects that cut it off can be simulated-- blindness, disorientation (getting lost). And effects that cut off the players' ability to interact with the game world are easy-- sleep, paralysis, petrification.
But that's about it. And you can see some of the classic effects are already huddling around the few, true ways to affect characters. In essence, the difference between sleep and paralysis to the pc is flavor.
The other thing a DM can do is apply modifiers to the system-- strength, speed, and pretty much anything that gives negative modifiers to combat. "Okay, you're drunk, you have +2 to strength, but -4 to hit."
But what happens when you want pcs affected by something else? This came up in one of my sessions when a magical statue made a pc paranoid. On the note I handed him communicating the effect I'd tried to apply the effect to the player rather than just his character. It said something like "Someone in the group wants to do you harm." But the player knew what was up. The only real way to simulate it was for his player to act paranoid. It was immediately apparent to everyone at the game table that he was under a magical effect and proper measures were taken. Yeah, it was sort of funny to see him act paranoid for a bit. But it seems like the potential for interesting things to happen is pretty limited by the nature of the game.
The nature of the game is that the players share a consciousness; any information one of them knows will tend to be known by all of them. And I'm not talking about "bad" players or players "cheating". Just the fact that we're all sitting around a table means it takes great effort by the DM to prevent everyone from knowing everything (individual notes, ear-whispering, talking in a separate room). And, even those efforts will tip the rest of the players off to the fact that something fishy is up.
This is not meant to imply that players roleplaying effects isn't or couldn't be fun. I'm just trying to think here about how much system can do for us, and if there are any ways we might creatively get around this hurdle.
Here are some effects that would be hard to simulate without pushing it onto players as a roleplaying responsibility:
- any kind of insanity, really
- memory loss
- Confusion could be crudely handled by making a chart of possible actions and having a player roll randomly each round.
- I just realized that the way the 1e DMG handled insanity was for the DM to take over roleplaying the character from the player, so no help there.
- Hallucinations and some forms of insanity could fit into the DM's parceling of game world information. It's just that the goblins fighter bob keeps hearing behind doors never materialize.
- Look for new ways to obscure/filter game world information.
- Look for new ways to mess with the shared pc consciousness.
- Think about ways to actually affect players in lieu of their characters.