I usually cosplay from things I know and love, but I can understand being pulled into a group cosplay of a series/game that you know absolutely nothing about. (My friends wanted me to cosplay Suki of Avatar, but I filled up my schedule up with cosplaying guys instead.)
At that point, I would at least heavily research the character and read/watch part of the the manga/anime if there is one if I could not read/watch it throughly.
I feel sort of bad because in our Nabari no ou group, we're recruiting my friend's little brother to be Miharu (the main character)... needless to say, I don't even know if he knows he's being forced into this. We'll probably make him at least watch part of the anime (or read the manga I have...)
We're also asking another friend of ours (okay, making) to be Yoite... he's only watched part of the anime (which doesn't count because the mangaverse is the REAL DEAL... the anime is just lies for the second half of it.) and I don't know how we're going to tell him... certain facts... about his character. :| AND the fact that fangirls are going to think of the two as a 'couple' might be sort of awkward as well.
OH WELL. :'D It's all for the sake of cosplay! (I'll make sure their cosplays are great, that they can pose amazingly, and can at least manage a decent conversation with a rabid fangirl.)
I feel kinda bad because though I love Prussia to death, I haven't actually seen him appear in the anime yet. o: (I can't watch anime online because I get distracted. Shiny, shiny internets.) Soon. Soon, I promise.