The professor who drinks from a flask
The Alastor Moody who teaches fourth year is Barty Crouch Jr, kept alive by Polyjuice. The flask, the paranoia, and the way he steers Harry through the Tournament are the disguise.
Setup
1Harry Potter and the Goblet of Fire · Moody’s first classes and the flask
The new Defence teacher drinks from a hip flask all day and behaves as if everyone might attack him. The school reads that as Mad-Eye’s known caution. He also nudges Harry toward the clues of each task.
Payoff
Harry Potter and the Goblet of Fire · After the graveyard
The Polyjuice wears off. The man in Moody’s office is Barty Crouch Jr. He put Harry’s name in the Goblet, turned the Cup into a Portkey, and used a year of Moody’s face to deliver Harry to Voldemort. The real Moody was locked in a trunk.
