I went and saw Harry Potter and the Order of the Phoenix today. Let me tell you, you can really tell the people who haven't read the books! Here are a few choice quotes I overheard today:
"What's a Gnargle?"
"Who's that?" (when Dudley was on screen)
"Is that a tornado?" (the arrival of Dementors)
"Is she a real person?" (asking about Mrs Weasley)
"Is Harry sad?" (Harry is looking out the window looking sad)
There were more, but I can't think of them right now.