“I Didn’t Know You Could Read” (Harry Potter and the Chamber of Secrets)

This one’s very surprising, especially if you consider actor Tom Felton was just 15 when he delivered it. The line “I didn’t know you could read” was directed at the Slytherin bully Goyle, who was really Harry Potter in disguise (he drank the magical Polyjuice Potion).

While the line fits very naturally with the character of Draco Malfoy, it was never part of the script nor of J. K. Rowling’s books. It was a sheer moment of inspiration from Felton, who’s proven to have a knack for improvising on the spot.