Interrupting people is one of the major difficulties of walking around conventions and asking for photos. Many cosplayers are looking to have their photo taken, but sometimes that is not the case. If someone is on a cellphone, resting in a corner out of the action, talking seriously to a friend, in the process of buying something in the dealers room, eating something, or not giving you any sort of eye contact or body language as you approach, it might be best to just walk out of the situation rather than putting forth the effort to ask. A lot of the reason why I do this is to make images for cosplayers that they can admire and enjoy. Dealing with people who don’t want to be approached or snubbing my request is not really worth dealing with. That being said, bad situations are very rare and most often people will completely drop what they are doing to pose for me. Just be prepared and look for individuals who are in a situation where interrupting what they are doing will actually make them happy rather than the opposite case.
Look for individual cosplayers walking around aimlessly, browsing in the dealers room, or just hanging out in high-traffic areas. Chances are they are ready and willing for a mini photo session. The thrill of being recognized for their work is a big part of why they do cosplay, so the more people like that you can photograph the better. By asking for a photo, you are helping them enjoy the convention and will possibly make a good connection for future conventions if you give out your contact information afterward. If you know the name of their character, saying it will sometimes get them to look your way, which makes things even easier. For groups, approach the person who is least active in any group conversation as they are the best ones to ask considering they are just standing around without much to do as their friends talk. Besides those two situations, there is often a guy/girl pair which are the more difficult people to approach. I can’t really give you any suggestions on how to approach that situation, but just to ask. Recently I had walked past a pair and wanted to ask for a photo of the girl in cosplay but did not. I had inadvertently given the girl eye contact, which I guess she picked up on. As I was taking a breather down the hall, they came back and stopped right near me. I took the opportunity to ask for a photo and in turn got the shots I was looking for. With good intentions almost any situation will work out for the best.