I was watching Crazy Stupid Love the other day, and when the son's talking about A-holes, the student behind him is completely mortified that he is talking like that in class. She really sells the scene.
In The Happening, Mark Wahlberg says something weird to a student, and the student behind him has the most befuddled look. It was so good that Cinemasins even removed a sin for it.
What are some other good performances by extras in a movie?
Edit: An extra is a background character without any lines.