I’m Not Going to Let You Do That
Typically, I don’t like to control the actions of players when they are roleplaying their characters. I try to encourage players to develop their character’s backstory on an ongoing basis, and share out loud the thoughts and motivations that their character is experiencing. However, there are a rare number of circumstances where I will challenge a player on a decision, and express my desire for them to do something different. Now, please understand, this is never done lightly; I once played with a DM who insisted my human character could not take any overtly hostile actions (given the time period of his homebrew setting), and when I tried to use one of my spells, he said that I should not cast it. I don’t like this level of intervention.
The first case would be if a character is going to attack the party, or do some other kind of impulsive hostile act. In my games, I have a rule that any character who betrays the party will be turned into an NPC; it’s not meant as a punishment, as the character is still functioning in the story, but I realize that it can feel very crappy for other players who were not viewing this game like a PVP thing. Any attack on the players will come from the DM only. For this reason, if someone’s character gets frustrated and says something like “ok well I’m going to draw my sword, and stab them in the neck if they won’t share that treasure.” or whatever, I’ll simply say “no you don’t.” Because I do this basically never, this really communicates how much I disagree with the action, but they also have the chance to insist, and tell me that they do choose that course of action (at which point they become an NPC). Sometimes I tell a player that they do not take the action they just said they did because it clearly shows that the decision goes against what the game is meant to be.
Another situation would be when a character acts completely outside their continuity they’ve been developing throughout the campaign. If a character has been, for example, generous to beggars every time they go into towns, but then decide instead to start killing them one session, I might stop them to ask why the drastic change. I will likely point out to them why the shift is so jarring, especially pointing out that they should consider the lasting psychological implications on their character going forward if they do something so drastically different. I give them a chance to explain the motivation behind the action, but if they says something like “I don’t know, I just want to.” then I will tell them no. I don’t think a player should be allowed to cause the story to become so disjointed all of a sudden, and ruin the narrative for the other players. That being said, if they insist, I might also just force a reason on them (dominated by a vampire? cursed by a hag? a family history of psychosis creeping up?). This is different if the player is new, in which case I might just ask questions about a decision to ensure that they understand what they are doing with their character’s story before allowing them to really make the choice.
Finally, the only other time I might tell a character they have to act differently is if they are magically compelled to and are not doing it right. Usually if someone is under the effects of a dominate spell I will still let them control their character, just giving them the instructions they have to follow. I also love to use the Crown of Opposite Alignment to force a character to suddenly act according to a different nature. In these cases where magic is interfering with their free will, I might have to step in and prevent a player from trying to circumvent the effects. Sometimes they think they can justify acting in a way that is not actually according to the compulsion because of some technicality. Unfortunately, this usually goes against the spirit of the affliction, and so I might take back control to prevent them from weaseling their way out of something that’s happening. Other than these situations, I cannot think of other good reasons to ever prevent a player from doing whatever they want with their character.