What to wear: The former king of King's Landing was always decked out in the finest royal garb, so see if you can find a robe or tunic in a shiny or brocade fabric. Don't forget to wear a short blond wig and a golden crown, and if you want to go the extra mile, carry around a gold wine glass and paint some fake blood coming out of your nose to mimic his iconic death scene.
How to act: Do you have an incredibly annoying younger sibling? Imagine their personality, mix it with that of a deranged sociopath or serial killer, and you'll have a vibe somewhat close to Joffrey's.