Yeah, agreed. I think in that situation... Let's say the Duke and the Baron are at the same party, and the players know that the Baron is Doctor Murder in disguise and the Duke is just a duke. If they're talking about poisoning the Duke, I'd probably edge right up to it without saying 'you have the wrong person'. "The Baron is remarkably calm. He's flirting with the woman on his left and his wife is glowering from down the table. It's hard to believe he's Doctor Murder under his filigreed circlet, despite the puppy you saw him kick before he got in his carriage." And then if the players go "Ok yeah how do we poison the Duke" well crap I guess I'm going with them poisoning the Duke...? I do want to preserve chances for them to make mistakes, and I don't want to say outright "I think you are doing the wrong thing." I guess I might ask why they're planning to poison the Duke and what their plan is. If it is a genuine OOC mistake, I want to clear it up, but if it's a case of the entire party just wildly missing the point I can't and shouldn't play their characters for them. It's a thin hair to split, I guess!