There are a ton of issues with that.
That doesn’t automatically mean it’s the wrong move, just that it’s fraught with problems.
Ultimately, most people feel that roleplaying works best when everyone, ya know, plays a role. As in, behaves as their character would, based on what their character knows, rather than how they would, based on what they know. This is usually the goal.
However, most groups don’t explicitly enforce it. It’s considered bad taste to meta-game, it’s considered good roleplaying to stay in character even when it hurts, but there aren’t specific rules about it, and in many groups the DM claiming “your character wouldn’t do that” is a gross violation of the player’s area of control (i.e. their character). Statements to that effect have been reasons to leave a group for a lot of players in a lot of situations, and while I am lucky to have never played under a DM who seriously thought that was his business, if I were I most likely wouldn’t tolerate it.
But your group may be different. You are expressing frustration with the status quo, and that is presumably a feeling shared by others. This could be a solution to that, and ideally the questioning would come more as a reminder than as any real attempt to control others’ characters.
That said, the objections of some people in your group suggests that not everyone feels the way you do. There are people in the group who either A. feel they are not metagaming, or B. feel that the metagaming is a good thing, and in both cases there is not a problem. Both perspectives are valid, though B is a bit unusual. (There is a third option, C, wherein people recognize that there is a problem but dislike this solution; I would probably fall in that category. That said, these people are probably already doing their best not to metagame.)
So what you really need to do is discuss metagaming, what is or isn’t and how much is or isn’t appropriate. You need to have a mature discussion, and you need to listen to others’ opinions, perspectives, and preferences. More than likely, no two people in the group will exactly align, but hopefully everyone will be near enough to some common ground that a compromise can be made.
And once you have that, you really probably don’t need this rule. You might include it, in theory, if people felt they needed to be reminded or “called on” for metagaming, but I really cannot imagine any point where it is a good idea for a DM to say “no, your character would not do that.” You can question an action (though even that might be disrespectful), but ultimately the DM has to back down there because his authority, so absolute otherwise, cannot control player characters like that, or else the players have nothing and there is no game.
Under no circumstances should this rule be even considered unless everyone wants it. A group that agreed it would be for the best to get called in this fashion could work. But if some do, and some don’t, it is not a reasonable thing for a DM to expect of players. The individuals who requested it could get called on it, but you should never tell someone he’s not playing his character right, after he’s specifically told you to stay hands-off on that subject. Were it me, I would walk out the very first time it happened, assuming you convinced me to stay at all, which I tend to doubt.
There can still be a compromise even if people object to this as a rule, of course; that’s actually normal for most groups. E.g. if you say something like “I won’t call you on it, but it is your responsibility to avoid metagaming and this game isn’t going to survive if you don’t.” and he says “OK, I will do what I can,” that is a workable situation.
But if no compromise can be made, if some feel that their behavior is entirely appropriate and refuse to modify it, and you feel your expectations are entirely reasonable and refuse to modify those, then you have learned this without going through it the hard way: you are not a compatible group of people who are looking for the same game. That’s pretty much what you’d discover if you tried to “enforce” these rules without a compromise, but there’d be a lot more ill feelings. Better to skip that step.
1. How do I discourage players using the knowledge that they have a low roll to influence character decisions?
Be up front and honest with them about not Meta gaming. It is meta-gaming using the knowledge of a low roll to influence your in-character actions/reactions/thoughts/etc. I usually say just what you said, something along the lines of, "You think she seems to be telling the truth." If my player then goes on to think she is lying, I do one of two things:
- Either gently remind them, "Your character thinks she is telling the truth and would not remain suspicious."
- Or, allow them to roll insight again, but on a different bit of information hoping for a higher roll so it's definitive that the NPC is truthful (or not).
The first option is always better, and usually in the long run makes for more engaging role playing when players can accept the cards their characters have been given. One major tip, when they roll low and you tell them they believe the NPC, try not to act devious or suspicious, your tone and demeanor can influence player thoughts tremendously.
2. Should I, and if so how, tell my players if somebody is actually telling the truth even if they have a very low roll to avoid them thinking they are being lied to and their characters are too stupid to figure it out?
This is a bit tougher... first I refer you back to the first part of my answer to the first question. Players acting on the knowledge of low or high rolls in a way that is discordant with the information you give them is Meta gaming. So, if the 2 options above don't work, then you have to ask yourself one question.
"How will having this player remain suspicious of the information affect the narrative?"
This situation is good for on the fly story development!
- The player remains suspicious, doesn't heed the warnings of the NPC and their actions either result in an extremely challenging fight, or worse may even lead to the death of the NPC. This could cause the player to be more careful in the future or become deeply regretful of his stubbornness.
- The player remains suspicious but the NPC pleads with them, "I can tell you don't believe me, and if I were you I might not believe someone like me either... but please good sir knight, proceed with caution!" This could also lead to the outcomes in the previous example if the Player still ignores the warning. Or they may uncover, incrementally, that the NPC was truthful, after which they might go back, apologize and make a new friend.
If the story really is better served with the character believing 100% that the NPC is not lying and there is no in game way to work around the suspicious nature of the player... as a last resort (and I personally would not do this), just tell them out of character that the NPC is being truthful. I can't think of a reason that you would need to have the character believing that the NPC was honest given the ad-libbed scenarios above, but if you find it imperative, it's your call to make.
...and nothing says the player won't suspect you of lying just because you are the DM. ;) Some players will always be suspicious no matter what as they will always believe the DM is "out to get them".
(And to quote another answer here after reading the stuff posted as I was typing this novel length answer... do be up front with your players that anything you say as a DM to the player will always be the truth. Lies might come from NPCs, but never the DM. And always make sure you are consistent in upholding these promises to your players.)
Best Answer
This entirely depends on your game, and your gaming group.
What you are referring to (acting as yourself instead of your character) is called "meta-gaming" and involves making decisions that are outside of the purview of your character's personality or knowledge. Whether or not this is an acceptable practice depends in part on your group and in part on the level or realism/seriousness of your game.
If your group is a lighthearted, we are playing this to have fun type group and you are playing in a system that focuses more on mechanics and tactics than on actual role-playing, then meta-gaming may be ok. You are likely free to make, at least in some limited sense, decisions that would not be likely/possible for your character to make, either because you personally feel a certain way, or because you have knowledge your character does not.
However, if you are playing in a more serious group, or more realistic setting where role-playing is the primary influence then it would be inadvisable to make decisions your character would not make, or act on knowledge your character does not have.
These are not discrete decisions either. Most groups fall somewhere in the spectrum between heavy story influence and heavy tactical influence. Systems do as well, although I feel this is more highly dependent on group dynamics and expectations than it is on the system you are playing in.
Sometimes role-playing your character leads to suboptimal (even abysmal) outcomes, this goes somewhat against our typical gamer mentality (I know it does mine), but is part of what makes RPGs special. You can have a ton of fun, even when your character dies horrible.
Talk to your group about what their expectations are. This can dictate a lot of how you role-play and how you make decisions. Does your group expect you to play your character within a certain set of boundaries (emotionally or mentally), or is this a no-limits, no holds-barred kind of group? This is something you can bring up with your group at the beginning of a session: "how much do you guys expect internal consistency on an emotional/knowledge level do you expect?" (or "is meta-gaming appropriate at this table?")
Remember that the goal of playing an RPG, most of the time, is not to win, but to tell a good story, whether you're role-playing your character or meta-gaming, remember this purpose.