When the heroes enter a combat and want to know information about their enemies (keywords, strengths, weaknesses etc) would the ability check require an action or is this a free action?
I have read Does D&D 5e have a rule for character knowledge about monsters? but it only discusses which skills to use, not what actions it would take.
My group thinks it should be a free action as you can ask your knowledge while you act. I think with 4 people in the group at least one always rolls high enough to get serious information. And of course it will happen at the start of every battle because it doesn't cost the PCs anything.
I am planning (if it's for free) to do either a group check or a passive check (like perception or insight, just with the appropriate skill), but I still think giving away information for free that can seriously change the outcome of the battle is quite cheap. Especially in Out of the Abyss, where you have many monsters with special features.
So – how does giving out monster information during combat work, action wise?
Best Answer
Remembering is not an action
Beholding a creature and trying to recall information about it would not take an action. Perhaps it might be considered "interacting with an object or feature of the environment" (PH 190) - which a character can do once on its turn without expending an action.
But there's another way to preserve a little mystery about your monsters…
Explanations are actions
If one character recalls the creature, that doesn't mean all characters have. You could slip a note to the character(s) who succeed the knowledge check. If they share more than "brief utterances and gestures" (PH 190) about the monsters, then it's rules-as-written to determine the character spent its action.
Player Knowledge vs Character Knowledge
If you've got power gamers at your table, it's also reasonable to insist the players don't abuse player knowledge. If the players have the MM memorized, and a player shares the monster's name, you can rule that the character spent its action giving the characters the knowledge the players already have.
(Alternatively, just change details about the published monsters yourself.)