I thought about making up an excuse to talk to all players in the
bathroom during stuff like the assassination example above so everyone
will be suspicious of each other but it sounds like too much hurdle.
Unfortunately, that's your answer.
Metagaming in this case isn't going to be deliberate, but it's going to be hard to avoid. If you constantly talk to one player and one player only, even perfectly honest players trying not to metagame are going to have a difficult time not seeing suspicious activity in anything that one player does. It's human nature. The biggest danger of it is that you give a metagaming speech and the players over-compensate by ignoring suspicious activity so to avoid the perception of metagaming.
The only way to avoid it is to either not talk to that player so often at sessions (by talking between sessions and letting the player improv as needed during sessions), or by talking to everyone so they have no reason to suspect any one person over another.
Example from my campaign
I recently ran a session with my players that was a peace negotiation. Everybody was playing an ambassador for one city (or nation), except one (he was playing his own character, as the host of the session). Every nation wanted something out of the negotiation. One of them wanted to see the whole thing fail. In order to ensure that nobody knew who that person was, I wrote half-page notes for every single player and handed them out at the start of the session.
The troublemaker knew who he was (because his note said so), but everybody had notes so nobody knew who was getting different instructions from everyone else.
Is that more work? Yes, absolutely. It was a lot of work. But it was a big success.
Other Tricks
If everybody has a phone or tablet at the table, you can exchange chat messages. If you do it rarely enough, it won't be overly noticeable. Doing it too frequently will make it obvious who you're talking to.
If you have one player come early, you can talk to that player before anybody else shows up. People don't arrive at the same time for games typically, so that's not overly suspicious.
You can hand the relevant player a note at the table with important information, at the time. But again, you'll have to do this with every player from time to time so it seems normal. I've done this with cases where one party member notices something odd that they might not want to share right away (like if only they hear a weird noise or think someone's lying).
If your players are playing their characters then you are a really lucky GM, and you should be proud of them.
But yeah, I understand. We've spent ages preparing an encounter for the group. We've gone over all their possible approaches dozens of times and put stuff to gently railroad them into the right places to cover every eventuality. And yet still, the group will never, ever stick to our mental script of the encounter.
The Malkavian, completely in character, throws a cake at our Seriously Badass Evil Villain. That wasn't in our mental script! Aaagh! Tell him off!
No. The player has just thrown us a bone, the ability to build on the actions of the character while at the same time showing the personality of our villain. We're a lucky GM indeed.
How does our villain react?
- Does he unflinchingly ignore the missile as beneath him, and let it bounce off and roll away unheeded, perhaps casually dabbing at the cream with a lace kerchief as he berates the party?
- Does the cake just dissipate into nothingness as it flies towards him?
- Does he block it effortlessly with his staff?
- Does one of his minions catch it from the air, crush it, and fling it contemptuously to one side?
The contempt the villain shows, the way the humor of the situation falls completely flat and unnoticed by him, will only build the villain's stature in the eyes of a player.
(If you know that you have a habitually-cake-throwing character, then you can prepare for it in advance! Instead of "that wasn't in my mental script!" you have the advantage, and can have a response lined up ready! I'd discourage too many OP disintegrations, though: most lower-levels would just wipe it off and say something like "Freaking Malks!". Only a fellow Malk would be amused or distracted by it... and THAT encounter would be a delight to play! "Oh my! Beetroot? Delicious! You MUST give me the recipe! Torturers! Take this one to the dungeon kitchens: I will have his secret!")
As GMs, it's not about us: it's about the players' characters, and about the players' fun. If they're having fun, great. If we're upset because our carefully-crafted atmosphere got ruined, then that's sad... but also just plain wrong-headed.
If we let the players have those characters, and then we place them in a setting in which they cannot both play those characters, and still have fun... then we messed up somewhere. Where?
- We let some players into our group who are just so mean and ill-spirited that they are out to ruin the game for everyone. Solution: let them back out again.
- We're just not doing our atmosphere-building well enough, so our sense of atmosphere is destroyed by nothing more than a flung cupcake. Solution: Ask whether our villain would let his thunder be stolen by a Malk cupcake? Yes? Then that's fine. No? Then get in his head and role-play him!
- We are caring too much about setting and atmosphere. We want, perhaps, the players to be hanging on our every word, instead of having fun. Solution: accept that if the players are having fun, we're already winning at being a GM! Even if they aren't enjoying themselves as we had planned!
- We let the players have the wrong characters for our campaign. Solution: apologize to them, explain what the campaign's about, and ask them to re-roll.
- We have created the wrong setting for this encounter, given the party and the players. Solution: play it through, learn from it, and change the setting for future encounters to work better with the party dynamic.
Best Answer
Many years ago, a member of the AD&D group I was in died in an accident. We took some time off, as a group, then got back together to play.
The first session, the GM brought each of the player parties we'd had together in a situation drawn from their previous adventures, an all-out battle with the missing player's characters at the forefront and the biggest, baddest evil we'd ever fought opposing. The intent, obviously, was to provide closure for us, the players, as the characters died heroically. In the one battle we won (due to an unexpected -- by the GM -- invocation of a setting-specific power by my own character, the only PC who could have done so in that situation), the missing player's PCs were translated into the heavens.
It's a bit different situation from suicide, but allowing the missing player's character(s) a heroic death with the rest of the group at their back seems a very good way to provide closure to the other players.