No you not need to know the other person well, you just need to know a little about the lore and background.
Again I bring together the acting and roleplaying. You not need to know the other actor well for play a character, but the main script, where the scene take place, when, what happened before, what are you doing what your character planned to do, the rest can be totally improvised.
OOC (out of character so without roleplay) on an MMO generally things go like that :
-You contact someone you know he is roleplayer (Discord, in game etc), propose a schedule for rp
-You must set the background needed for roleplay. Where it take place, when, how and why your characters encounter and evantully talk a little more for know each other preference of roleplay.
-After that the rp (In Character) start and let’s go your character talk, not you, for example if you play a paladin, your character will not miss an ocasion to save the orphan.
On private server 100% rp, you not need to talk OOC before roleplay. It can start without any consent because if the other player is here it’s for play his character now in the actual condition.
So before all if you are interested to rp, you must choose a game or a social network, what you prefer between both? The second choice is text only, it’s not game.