Zhaleskra
Adventurer
Personally, aside from the fact that "prone" means lying on the ground in a specific position, I'd go for "creatures that are already 'lying down' can't be made to lie down". So this is one case where I'd tell the rules to Sit Down and Shut Up for the sake of the believability of the game.
Regarding changing monsters, I could justify with "if you're using metagame knowledge, so will my NPCs". Of course that's a last ditch if players are just "going textbook". Perhaps my troll is a scandinavian troll which do have different weaknesses from standard D&D trolls.
Regarding changing monsters, I could justify with "if you're using metagame knowledge, so will my NPCs". Of course that's a last ditch if players are just "going textbook". Perhaps my troll is a scandinavian troll which do have different weaknesses from standard D&D trolls.