In Harry Potter and the Goblet of Fire, we are introduced to a new character named Alastor "Mad-Eye" Moody. He is the new Defense Against the Dark Acts teacher at school and is nicknamed "Mad-Eye" because of one of his enlarged eyes (in the film, it's an eye on a strap).
We later discover that the Mad-Eye Moody that had been teaching them for almost a year was actually evil Death Eater, Barty Crouch Jr. (portrayed by the guy that played the Tenth Doctor in Doctor Who), who had kept the real Mad-Eye Moody locked in a trunk for nine months.
Harry had seen Barty Crouch Jr. before. He had seen him in a dream along with Voldemort and Wormtail and he had also seen him when he looked into the Pensieve. While looking in the Pensieve, Harry sees a young Barty Crouch Jr. arrested and convicted for the torture of Frank and Alice Longbottom. And one of the most noticeable things Crouch does in this scene is a tongue twitch.
Fun fact: that was improvised by David Tennant.
He appears to do this when he is threatened and also does this when his Moody disguise wears off.
But he also did this WHILE he was impersonating Mad-Eye Moody.
In the scene after the Second Task, Barty Jr's father, Barty Crouch Sr., is talking to Harry. Barty Crouch Sr. is the one that imprisoned him in the first place and when Barty Jr. went up to him under the disguise of Moody, he does the same tongue twitch to his father.
After that, Barty Sr. recognized the tongue twitch and had a look of recognition on his face.
He remembered the twitch Barty Jr. did and I'm not sure if anyone noticed Moody's twitching but this is direct foreshadowing to the fact that he was actually Mad-Eye Moody in disguise.
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