"You found a cursed ring!" How do I let the player roleplay it without spilling the beans to everyone prematurely?
In a game I was running, a player tripped a magical effect that would alter his behavior (failed save against compulsion magic). I took the player aside to explain, out of game, what had happened -- this would allow the player to roleplay and, I hoped, would not be as obvious to the other players (whose characters had not detected the effect yet). I wanted my player to play normally but apply the compulsion to the character's actions, in other words.
Of course, there's an obvious problem: the other players had the meta-information of this interruption in play and our private conversation, so they were suspicious of the player (and character) immediately, and had trouble suspending disbelief and playing as if they didn't know something was up.
How could I have handled that better? Should I have had a conversation with the group about what was happening, with the expectation that they'd all roleplay it? That removes some of the surprise and leads to questions of "when would my character reasonably figure out something's up?". Should I have anticipated the problem and prepared a note in advance to pass to the player if needed? (Should I then have prepared other decoy notes, so that my passing a note wouldn't by itself provoke suspicion?)
How, in short, can I help the group reduce the effects of meta-game knowledge so we can focus on the story at hand?