Immersion versus metagaming.
Verisimilitude versus very smug attitudes.
When meta makes it better:
Players making choices for the benefit of the game rather than being completely true to their character. For example, foregoing summoning spells or follower feats to avoid slowing the game down or taking significantly more time than other players in a large group, or traveling with the rest of the party when they’d really rather be questing alone.
When metagaming is questionable but still serves the greater good:
Player characters being more tolerant of companions’ behavior (or alignment) than they would of an NPC’s in order to maintain party cohesion.
One PC reminding another of their abilities when they seem to have forgotten. It helps to dress up suggestions as in-character, such as a barbarian asking the wizard if he might consider casting a particular spell effect rather than outright naming it, especially when an experienced player with a character of low intelligence or wisdom is trying to assist a newer player whose character has high mental attributes.
A player recognizing that a foe or challenge might be too difficult to engage (accompanied by a request to allow a knowledge check for their character, or a nudge to another character to make one).
Metagame knowledge used in puzzles. As mentioned in a previous post, the GM needs to decide and communicate beforehand how much meta knowledge use, if any, is allowed or expected in solving a puzzle or riddle.
When metagaming is undesirable:
When a party member is making decisions, that, while tactically sound, are completely out of character (classic examples include a berserker being unusually cautious rather than reckless, or a paladin letting helpless innocents die instead of trying to save them).
When a player is making too-thorough use of real-world book knowledge instead of their character’s in-game knowledge (whether it’s a monster’s specific abilities and defenses, the complete content of a published module or adventure path, or a copy of the GM’s personal notes obtained with a telephoto zoom lens while hanging from a zip-line above their office window the previous night).
When party planning and discussion is more out of character than in character.
Other immersion-ruiners include constant real-world references or comparisons, joke names (was Discworld really that funny?) and generally inattentive behavior.
Finding the balance for your group:
Pregame preconceptions – put everyone on the same page to avoid issues with trust and communication. Establish what kind of basic knowledge characters can expect to have. You will control the flow of information, whether it’s through an encyclopedic primer or a slow trickle of revelations for a campaign of exploration and mystery. Having players constantly stop and ask if their characters are allowed to know something can break immersion as much as metagaming might do.
Pregame meta – a collaborative approach to the initial party construction for compatibility (in class, skills and alignment) is perfectly okay.
While ideally a player character party should have a broad range of knowledge skills, sometimes they have none. If there are foreseeable issues with player knowledge outweighing character knowledge, consider distributing bonus knowledge skills through background traits, or crossover common-sense knowledge based on related skills. If things look especially dire, consider introducing minor divination magics such as an identify spell for monsters as well as magic items.
Will a character’s role-playing be undesirable if it goes against sound party tactics? Generally anything that stays true to the campaign world and aids in immersion is to be encouraged, but there may be situations when one character’s actions, however justifiable, might spell doom for the rest of their companions and bring a premature end to the campaign. You might want to avoid that.