Answer: At this year's Sci-Fi Weekender in North Wales.
I was lucky enough to get invited to this event as a guest author with an access-all-areas "artist" pass and the chance to witter on at great length on stage about genre writing - and my genre writing. You can read more of this side of the event on 6th April in my monthly blog over at AuthorsElectric, but here I thought I'd talk more about cosplay and role-playing in general.
Up until recently, I'd never even heard the term cosplay. While I've been to (and organised) many conventions over the years, they've been more aimed at the literary and art side of the science fiction, fantasy and horror genre - more concerned with discussion, readings and art shows. This is the first event I've attended that celebrates the shows themselves, where 6000+ attendees turn up to meet stars and dress up as their favourite characters. I used to do a lot of live role-play (of the Treasure Trap variety), but other than a brief and uncostumed appearance at a Star Trek convention in the early 1990s, I've never experienced such a large group of people having so much fun with costumes before.
Witness Peter Capaldi's new incarnation of Dr Who arguing at great length with a dalek over which of them is the bigger star and has the biggest BBC contract; Pennywise the clown wandering through the arena menacingly, with his clutch of balloons; a blue-skinned woman browsing books in the dealer room, Willy Wonka watching a panel or Obi-Wan Kenobi doing battle with Darth Vader by the outdoor bar.
And that's not doing homage to the steampunk crowd in their beautifully-detailed Victorian outfits. Men and women - whole families having a weekend away from television, playstations and the internet, and mixing with like-minded people. I could have stood outside all weekend just watching the interplay, the impromptu re-enactments of scenes from film and television.
In the trading area, I was helping to man the Telos table. The stand across the way had a whole wall of artwork - mostly by the wonderful fantasy artist Rodney Matthews, whose posters papered my walls in my student days (who remembers Athena posters in the early 1980s?). And I had a complete fangirl moment when I realised that the man himself was there, helping to run the stall! So I had a chat with him and bought a signed print. Totally made my weekend!
Would I go again? Definitely. Not sure I'd dress up myself. I did feel sorry for the cosplayers on occasion - everywhere they went, they'd stop to pose and have their photograph taken. But I guess that's what it's about for some of them - the appreciation of the onlookers into the sheer amount of effort it's taken to put some of these outfits together and the character-playing throughout the entire weekend. Hiding behind a persona can be fun in that you can do all sorts of things the real you would never dare to do! Which is kind of what we do when we write isn't it? Put on another identity and experience life as somebody else.
Edited to add this....