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Language: There is generally no need to go all verily foresooth, et cetera. So long as you try to avoid modern slang, regular language is perfectly acceptable – including curse words and figures of speech.
Meta-game/Meta-gaming: Knowledge that your PC couldn’t reasonably possess. As a general rule, players should try to ask themselves whether it is reasonable that their PC would know a given thing. E.g. I know roughly how a catapult or napalm works, but unless my PC has experience or some Engineering skill, they wouldn’t have the faintest idea.
NPCs Are People, Too: NPCs are ‘real people’ just like the PCs – they just happen to often be lower level or literally class-less persons. Treating them like cardboard cut-outs or robots will not cause people to like your PC much, generally-speaking. They will try to stay alive, bleed if you cut them, yadda yadda.