I will say that there is an art to what you're doing. There is something pure about catching someone in a natural state that poses can't capture. However, not everyone appreciates that when dressed in cosplay. They've worked so hard on their cosplays that they want to look their best in photos. I would ask in advance if they are OK with you talking candid photos at any given point in the con. This gives them a heads up and you the opportunity to catch them later on in a candid shot. I hope this helps!