At what point...

In the books, the Weasleys come to collect Harry to take him to the burrow ahead of their trip to the quidditch world cup. While there, Arthur is called away because of a disturbance at Moody's place. He supposes that Moody saw a cat and mistook it for an assassin, or something similar, which introduced the reader to Moody's paranoia. We later learn that this is when Crouch Jr attacked and took his place.

That part of the book didn't make it into the film because of time constraints etc, but the viewers who aren't familiar with the book are just supposed to assume, I think, that it occurred before we meet Moody for the first time.

"You'll find it's a very small universe when I'm angry with you"

it was the whole time

I always wondered about this. If it was Barty Crouch from the beginning then why would he teach the kids how to fight against dark magic? Why would he be nice to Neville? I know dumbledore wanted him to teach them that but he didn't have to be so good at it.

I was just about to ask that. I think from the start.