I like to direct cosplayers. It's nice to have someone who knows how to pose, but I also like not to have the same photos as other photographers ; so I often suggest poses based on what I know about the characters I shoot, and if I don't know the character I ask the cosplayer about defining character traits, in order to suggest fitting poses and capture the right expression.
For me it's all about dialogue, and making the experience unique for both of us.
There's this other photographer I've been knowing for some time, who always trusts the cosplayer to get the pose and expression right by himself. His photos are technically good, but they rarely fit the characters. He may be a good portrait/fashion photographer, but to me he's not a good cosplay photographer.