My first shoot with the DC Cosplayers happened to be Halloween themed . . and it kicked ass.
I was floored with how cool some of the costumes were and how cool ALL of the people were. They are very careful to admit the same numbers of cosplayers to photographers so we can all spend 5-10 minutes with each person.
That way, the Cosplayers get to work with different photographers and change up their style a little bit between shoots, and vice versa. Some of the photographers are fairly new to the craft and some of us are pretty seasoned and it's a nice way of letting everyone gain more experience.
I don't naturally do a lot of posing when I work with models so it was a little bit of a challenge for me to learn how to do so as many of the cosplayers will just wait for the photographer to tell them where to go, how to stand, and what to do. I've done it a few times now and am a little more confident . . . though I have a lot to learn.
One of the most important things is to start by asking the person about their costume, especially if you don't know who/what they are. Figure out how their character moves and what aspects of their makeup or costume they want to emphasize. ESPECIALLY if they made all or parts of the costume themselves. You want to get lots of hero shots of those items.