Uncle Vernon: And Dudley, you will be?
Dudley Dursley: I'll be waiting to open the door.
Uncle Vernon: Excellent. And you?
Harry: I'll be in my bedroom, making no noise and pretending that I don't exist.
Dobby: Harry Potter! Such an honour it is!
Harry Potter: What... Who are you?
Dobby: Dobby, sir. Dobby the house elf.
Harry Potter: I see. Not to be rude or anything, but this isn't a great time to have a house elf in my bedroom.
Harry Potter: It's all right Ginny, It's over. It's just a memory.
Lucius Malfoy: Let's hope that Harry Potter will always be around to save the day.
Harry Potter: Don't worry, I will be.
Ron: Where's Hermione when you need her?
Draco Malfoy: Scared, Potter?
Harry: You wish.
Ron: Follow the spiders? Follow the spiders?! Why couldn't it be "follow the butterflies"?
Ginny: Mummy, have you seen my jumper?
Mrs. Weasley: Yes dear, it was on the cat.
Arthur Weasley: Now, Harry you must know all about Muggles, tell me, what exactly is the function of a rubber duck?
Draco Malfoy: My father did say this; it's been fifty years since the chamber has been opened. He wouldn't tell me who opened it, only that they were expelled. The last time the Chamber of Secrets was opened, a mudblood died. So it's only a matter of time before one of them is killed this time. As for me, I hope it's Granger.
Mrs. Weasley: Your sons drove that enchanted car of yours to Surrey and back last night.
Arthur Weasley: [to the boys.] Did you really? How did it go?
[Mrs. Weasley hits him.]
Arthur Weasley: I mean, that was very wrong indeed boys. Very wrong of you.
Moaning Myrtle: Oh sure, let's all throw books at Myrtle, because she can't feel it! Ten points if it goes through her stomach, fifty if it goes through her head!
Moaning Myrtle: Oh, Harry? If you die down there, you're welcome to share my toilet.
Harry: Uh... Thanks, Myrtle.
Neville Longbottom: Why is it always me?
Tom Marvolo Riddle: Haven't I told you? Killing Mudbloods doesn't matter to me any more. For many months now, my new target has been you.
Gilderoy Lockhart: Hello! And who are you?
Ron: I'm Ron Weasley.
Lockhart: Really. And who-who am I?
Ron: Harry! Lockhart's memory charm backfired. He hasn't got a clue who he is!
Lockhart: This is an odd sort of place, isn't it. Do you live here?
Ron: No.
Lockhart: Really-? [Ron hits him on the head with a rock.].
Dudley: Who'd want to be friends with you?
Answer: Yes. It's a common occurrence in a lot of movies for actors to forget their original lines so they improvise something to help move the scene along and not break character. Tom did this because he forgot what his original line was.