I'm not talking about a real Discworld Con.
The game would be set at an imaginary Discworld Con.
Then non-players can just come up and speak to you at anytime eg;
"Oooh look! There's that Pact-Breaker Sjoerd. My God, he's tall"
"Disliking Carrot would be like kicking a puppy."
"You kicked a puppy," Lobsang said accusingly.