Harry Potter 7

Interesting fact: In book 5, Harry receives a letter from Mafalda Hopkirk,a ministry worker who is never seen. In movie 5, this letter speaks, and Jessica Stevenson does the voice. And in part 7, Harry, Ron, and Hermione have to sneak into the Ministry of Magic disguised as employees. Ron and Harry choose two random workers, and Hermione chooses Mafalda. Since Jessica Stevenson might play the character in the flesh, they should get Simon and Nick Frost to play the other two guys. So we'd get Jessica, Simon, and Nick playing Hermione, Ron, and Harry in disguise.

And for the sake of playing against type(since the actors will have to imitate the three kids), Nick should play the guy Harry disguises himself as.