Gorgon Zee
Hero
I assume you mean one player trying to control the actions of another player's character, using game mechanics to do so.
The simple answer, as it always is: "I like it if it makes the game more enjoyable for the players". Everything else is an elaboration of this central goal. Things to consider in deciding if it makes the game more enjoyable are:
Group Dynamics: I personally am quite happy for another player to make some sort of influence roll and then narrate my character for a bit. My regular gamers are not liable to abuse this and so I feel quite comfortable with it. At a Con game, I might be a lot more nervous. I also fall into the "straight old white male" demographic, so my real-world experience does not have a lot of bad experience of others controlling my fate. This is not true for many people, so as a GM I need to make really sure that players trust each other before allowing one player to control another's action. I'd be especially careful when the player-dynamics are imbalanced, so I'd be more likely to allow a kid to control an adult's character than vice-versa.
Consistency: If you establish a way of handling it (for example, making the controller ask the GM to make the character act -- so establishing the GM as a mediator) try and do it the same way all the time. But at the same time, don't allow a situation to be un-fun just to preserve consistency.
Duration: Long-term strong control is a terrible idea -- it removes a player from playing the game. One player might be OK with their character being completely "dominated" for a few rounds (in an F20 style game); another player might be Ok with a character having an ongoing mild form of control (e.g. adding an aspect "I want to do what Siouxsie wants done" to their character).
Overall, my suggestions would be:
[1] The first time this comes up, break play and discuss it as a group, paying particular attention to quieter players
[2] Make it clear that the controller's job is to make the game more fun for the other player -- it adds responsibility to them
[3] Mediate all controlling actions yourself, so you can step in smoothly and fix issues.
The simple answer, as it always is: "I like it if it makes the game more enjoyable for the players". Everything else is an elaboration of this central goal. Things to consider in deciding if it makes the game more enjoyable are:
Group Dynamics: I personally am quite happy for another player to make some sort of influence roll and then narrate my character for a bit. My regular gamers are not liable to abuse this and so I feel quite comfortable with it. At a Con game, I might be a lot more nervous. I also fall into the "straight old white male" demographic, so my real-world experience does not have a lot of bad experience of others controlling my fate. This is not true for many people, so as a GM I need to make really sure that players trust each other before allowing one player to control another's action. I'd be especially careful when the player-dynamics are imbalanced, so I'd be more likely to allow a kid to control an adult's character than vice-versa.
Consistency: If you establish a way of handling it (for example, making the controller ask the GM to make the character act -- so establishing the GM as a mediator) try and do it the same way all the time. But at the same time, don't allow a situation to be un-fun just to preserve consistency.
Duration: Long-term strong control is a terrible idea -- it removes a player from playing the game. One player might be OK with their character being completely "dominated" for a few rounds (in an F20 style game); another player might be Ok with a character having an ongoing mild form of control (e.g. adding an aspect "I want to do what Siouxsie wants done" to their character).
Overall, my suggestions would be:
[1] The first time this comes up, break play and discuss it as a group, paying particular attention to quieter players
[2] Make it clear that the controller's job is to make the game more fun for the other player -- it adds responsibility to them
[3] Mediate all controlling actions yourself, so you can step in smoothly and fix issues.