How can I get a player to accept that they should stop trying to pull stunts without thinking them through first?
To explain the issue, one of the players in my game wants to assassinate a high-ranking government official. I have mentioned several times the government official is surrounded with guards and has powerful retainers. He keeps mentioning these plans that won't work to me; when I say no and explain the reason they won't work, he just keeps adding new things stealing from the session time while the rest of the group is trying to continue with the quest.
This isn't the first time this has happened as he keeps making plans that are not really well thought out and delaying the game. I tried telling him that he should think before asking to do something, but he just tells me that he did. I don't wish to just say:
"No, you are not doing that. You are going with the group." or "Your plan fails and you are caught before being killed by the guards. Make a new character."
How can I make him understand that he should actually think things through, instead of just trying to do what feels cool?
To answer the comments.
I am concerned with both derailing the plot and sapping the session time.
I would prefer to not to punish them too harshly for their actions. They don't mean anything by it and i am not running a game where players die every session.
I let my players fail a few times though i tend to bump up DC's with each failure so that it doesn't seem like failure has no consequences.
Best Answer
Clarify the situation
Whenever my players propose a plan that will in no way work, I first assume that I didn't describe things well enough.
In this situation, the player had a different image of the situation in their head. Maybe I forgot to mention the guards or the torches in my initial description. Either way, additional information allowed the player to adjust their plan.
What happens if the problem with a plan is something that the player doesn't know? What if every member of the royal guard has truesight, allowing them to see invisible creatures?
When giving out information, there are a few options.
Involve the party
When a character is about to do something crazy, it's important to remember that you control the flow of time. Events do not have to transpire right when a player says they do.
Pausing the flow of time gives the other players a chance to involve themselves, even if it just means leaving before everything blows up. If the other players are on board, you've turned "that one player is running off again" into "cool impromptu heist".
If you've clarified the situation enough and multiple players are discussing their options, it may be time for you to leave the room. (Thanks to Skrrp in the comments for this one.) I'll often take a break when a plan has potential, but still needs some ironing out. Like it or not, Dungeon Master is a position of authority. With you out of the picture, the only people to say "no" are the other players, and sometimes it's easier to accept "no" from a peer. Your return can also mark a logical moment to bring the discussion to a close. "So, what's the plan?"
Step through the plan's execution
Getting on the same page when it comes to a plan's details is difficult, so I've found that it's easier to take it one step at-a-time. After a long-winded plan explanation, I often find myself telling the player, "ok, so what action are you taking first?" If the player is just throwing ideas out, they'll often back down with a "well, I was just thinking". If so, then problem solved. A crazy plan is only crazy if you execute it.
So you've clarified things, involved a party member or two, and the doomed plan is still being executed. Now what? Is it time to tear up some character sheets?
In my experience, most seemingly-deadly situations can be made less deadly with a few tweaks. It's easy to say "the guards spot you trying to sneak into the throne room and you die in a hail of crossbow bolts", but that's not very fun. My go-to solution is to have the plan fail at the earliest (or safest) opportunity.
If the player was caught stuffing important documents into their pack, it would be much harder to justify letting them off easy. Instead, the plan failed at a point where the party has an out. They may come up with a creative lie, the bard may charm a guard or two, or the fighter may bribe the guards and apologize.
There should be some consequence for failing, but that consequence shouldn't always (or usually) be death. Whenever possible, the consequence should move the story forward, rather than grind it to a halt. (This concept of failing forward is used in Dungeon World, a more narrative system, but applies to 5e as well.)
Other considerations
None of these steps involve telling a player "no". If I have clarified the situation, involved other party members, and the player has reached a fatal step in the plan's execution, I still don't say "no". Instead, I say, "Alright. Describe for me how your character dies." (I ask this right before the point of no return. If a player backs down here, I allow them to do so.)
That doesn't mean that you should never say no, but that you should reserve it for special cases. If a player says, "my character kills the annoying child", and your group has an (explicit or implicit) agreement not to harm children, then "no" is the appropriate response.
My answer thus far has assumed a player acting in good faith. Stepping through the plan's execution can help weed out the trolls ("I run at the guard that's pointing a crossbow at me and stab him in the face"), but it's hard to be sure. If a player is intentionally breaking the game, it's time for an out-of-character conversation.