Jason Isaacs, the actor who plays Lucius Malfoy, relates a great story I hadn’t heard before about how a memorable exchange in Harry Potter and the Chamber of Secrets was improvised:
I remember my very first day, I improvised a line. … I had to kind of flounce out of a room when Dumbledore, played by the late, great Richard Harris, put me in my place, and there was no line written, no exit line. And I’d been humiliated, and my plan had come to nothing. And I said to Chris Columbus, “Don’t you think there should be a line?” And he said, “Well, say something. Say whatever you like.” So we did another take, and I hadn’t told anyone what I was going to do. And as I turned to leave, I looked at Daniel [Radcliffe], and I said, “Let us hope Mr. Potter will always be around to save the day.” And then Daniel, who was all of 12, stepped right up to me, looked me right in the eye, and said “Don’t worry. I will be.” A chill went down my spine. And as he did it, I thought, “Christ, this kid is good.”