Twisty the Clown From American Horror Story: Freak Show
What to wear: Get a white clown costume and dirty it up by rolling on the ground a few times. Maybe even splatter some fake blood on it for effect, and you don't need to mimic the terrifying sight under Twisty's mask, but you should see if you can make your own mask.
How to act: This is one of the scariest costumes you can go with this year, so play it up. Don't speak much, and just creep on people.