At the end of Goblet of Fire on the train back to Kings Cross the trio are sitting in the compartment when Hermione tells Harry and Ron about Rita being an unregistered animagus. She then shows them the jar with the beetle in it. I am curious to know how Hermione managed this.
I doubt that Rita willingly transformed into an animagus at Hermione's polite request, even if threatened with being exposed as an unregistered animagus.
My thoughts are that Hermione would have had to find her somewhere in the grounds already as an animagus and catch her, but then how does she have the conversation she alludes to when telling Harry and Ron about it on the train? It would have to be a two way conversation I should think, so how could Rita respond if she was a beetle?
I doubt there is anything in canon, but does anyone have a different theory?