Ah, power playing… A common issue among roleplayers. Sometimes it's because people are new and don't know any better, other times it’s because people have gotten accustomed to doing it that way. And occasionally it's simply because that player felt like doing it (either for that situation or just whenever they want.) Whatever the reason is, power playing is an issue and it needs to be addressed. But before we can fix it, we need to know what it is…
In short, power playing is when a member directly controls another member's character without that member's permission. This can be anything… As small as moving a cup on a table to speaking to using the other player's character in a heated gun fight. As long as you didn't have permission to use their character, it's still power playing.
For example, in this case Roleplayer 1 played Steve and they wrote:
Steve sat down across the table from Dan. Steve grabbed his menu from the rack in front of him and quickly opened it up. “So, Dan, what's good on the daily special menu today?” Steve asked. “Uh… not much today. Easiest just to order off the normal menu. They make pretty good burgers here.” Dan replied.
Roleplayer 1 has just power played Dan. They didn't allow Dan's owner to respond at all, effectively forcing Dan's owner to either accept the forced usage or to reject it and/or correct the issue.
Power playing should be avoided because it ruins interactions. Some of the major reasons that power playing ruins interaction being that:
Power playing doesn't give the other player any chance to react or introduce new material of their own. Instead power playing forces other participants to accept actions their characters may not want to do and/or moves their characters in ways they would have moved them anyway. Which makes it harder for them to post next time since they have basically had their post already composed and posted for them.
A side effect of this is that it can lead to premature thread death since in most cases double posting is not acceptable and the powerplayer has essentially already taken the other player's turn by deciding their actions and left them with little, if anything, they could do. (Unless they decided to power play in return…)
If the person who's character was power played (EX- Dan) decides to undo the power play then it can make some nasty confusion for anyone reading the thread. What really happens where? Say from the example earlier, what if Dan's owner wanted Dan to say:
“The chicken linguini is pretty good. Oh and did you hear about last night's game?”
So now if Dan's owner makes a post and corrects the situation then it may become very difficult for others to follow. Say Sally's owner comes along and reads the topic hoping to bring Sally into it, but sees that Steve said X and Dan said Y but the stories don't match up and don't make a lot of sense… So Sally probably won't join the thread.
People often don't like others using their characters without permission. They may have spent a lot of time crafting that character to their liking. They may have a particular mindset for that character. And anyone else who uses the character may not get it right. (Since it is their character they would know if the character is acting as they should.) And thusly they may resent having someone else use their character. (Especially in a way they feel is not accurate to how they would really act.) So then the owner may decide not to enter threads with the powerplayer anymore or worse… Leave the forum if the power player is left unchecked.
And of course if corrections happen, then it may cause the person who originally power played to feel as if they just got their character power played or had their post ignored. So they may either further power play or end up leaving because they feel like they're being ignored.
Another issue with power players is the community may decide not to roleplay with them if they power play since they don't want their characters to be controlled. They may not make threads with the power player and if the power player join their threads, they may ignore power player's posts.
Staff may end up having to be called in to resolve situations with power players since power playing is usually against the rules of the forum. And no admin really wants to have to tell a member that they can't do something, it makes the admin feel like the villain. (Even if they're in the right in telling the member to behave.) Also, no member likes to be told that their behaviour is unacceptable. They may resent the admin for telling them to behave or at least feel like they're being unduly picked on.
If there is an outcome you REALLY want… Talk to the other player first and see if they'd be interested in it too. It's no use trying to force an outcome on them if they're not interested in it. (All that will usually happen is one member will exit the topic, it'll die and nothing further will happen. Or it may get written off somehow. Even if you get the ending you\re after, it may get ignored.)
Otherwise the way to avoid it is to simply leave room for the other players to act/react. Don't make responses for the other characters. (Yes, it really is that simple.) No matter how much you may want their character to say/do <X> don't force them to. Let that roleplayer make up their own mind about what they want their character to say/do. If they like where it's going they might respond as you want. If they don't like where that topic is going they make introduce a new issue that you may like better than your own original idea. And that is part of the fun… Seeing what others come up with and responding to it.
If all you're after is your own characters reactions then why not simply write fanfiction? Or your own stories?