It can be harmful. It can also be helpful. Depends on what's being metagamed and how.For example, I have no problem with the 'I'm strangely open to trusting this complete stranger because he happens to be a fellow PC' effect, because it improves game flow and gets the new character into the game faster.
If I have to ask the GM for it, then I don't want it.
For any decision or adjudication, ask yourself, "Is this going to be fun for everyone?" and "Is this going to lead to the creation of an exciting, memorable story?"
DMs: Dungeon Master 101 | Session Zero | Structure First, Story Last | No Myth Roleplaying | 5e Monster Index & Encounter Calculator
Players: Players 101 | 11 Ways to Be a Better Roleplayer | You Are Not Your Character | Pre-Gen D&D 5e PCs
Content I Created: Adventure Scenarios | Actual Play Reports | Tools | Game Transcripts
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...I think the whole concept is the last redoubt of the fantasy realists. It's a problem that exists only in their minds because of limited thinking. It ruins their "immersion" if something doesn't line up in the fiction with the one way they can imagine it....
The character is right there in the combat. He knows the difficulty of fighting the monster and is determining tactics against it. That translates to the player via knowing the AC and calculating Power Attack.
Metagaming is rarely in issue in my group, mostly because we are all fairly immersive players who all think that abusing too much metagame knowledge can ruin our own fun. This is an important agreement, as we players have pretty much all read every 3.5 book we can get our hands on cover to cover and all have an enormous amount of metagame knowledge to not use. There are big bits of knowledge that are easy to "forget" about, but some of the finer points often slip through the cracks of play. Thankfully, this thread has actually helped me to exorcize some of my own worries that we don't do enough to persecute and destroy those guilty of incidental metagaming. navar100's comments and mvincent's response in particular have given me some great insight into our own style of play. I never really thought about it, but that is how I look at metagaming. I'm afraid I have nothing to contribute but thanks. So, thanks.
and then designs a toon specifically to counteract that module.
Toon?It's another name for Character. Its interchangable, but sometmes people have a negative connotation associated with it, suggesting that the character is really 1 dimensional and never shows any advancement.