Trick or Treat is a game of fast paced, quickly changing interactive scenes. It is ideally used to hone improv skills and thinking on your feet. The encounters should happen quickly and only last a minute or two. Trick or treat is not a serious game and should be taken in a lighthearted manner.
Trick or Treat is a horde game. A horde game has a core set of player characters and a horde of non player characters. The NPCs come and go, but the PCs remain throughout the game.