Jack messing with Bellatrix further: "Aww, calm down, crazy kitty. Maybe you and I should go somewhere quiet and work out a few of your anger issues."
Ianto constantly snarking, and when he finally learns the Silencio spell, much to Owen's discomfort.
Jack complaining to Draco and Harry that they're acting like a pair of jealous girlfriends about him. Also his various insinuations of UST between the two.
Jack and Gwen manipulating Skeeter into being fired by talking about a lot of false gossip such as Harry and Snape getting together and other hilarious things.
Ianto snarking (interaction paraphrased):
Hufflepuff Firstie: *about two slytherins acting strangely about her* Dunno. Two big tough kids. All muscle and no brains.
Ianto: You just described their entire Quidditch team.
In chapter 56, Jack and Ianto are playing around with the Room of Requirement and Jack wishes for Umbridge's clothes, letting her keep her underwear. Cut to:
Dinner in the Great Hall had suddenly become a great deal more interesting.
Owen and Ianto's Quidditch Commentary. All of it.
Owen: And on the Slytherin side, we've got Ogden, Stroulger, Urquhart, Travers, Crabbe, Goyle and Harper. So... that's five and a half mountain trolls, and a name-thief.
Harry goes to ask Jack for help getting a date that isn't psycho. Cue offers of poisoning, Nott having an "understanding" with Luna that would allow her to date Nott and go out as friends with Harry, and a discussion of Owen poisoning Wizarding minds by teaching them dirty Muggle words and everyone blaming Owen for everything.
Zabini: That understanding being that she's crazy and in three months' time you'll technically be a paedophile?
When Snape uses the word 'master':
Draco: We don't use that word in the dungeons anymore, Professor. Jack maims people for it.
This exchange in Chapter 80 is pure gold:
Draco: So put me in detention! Report me to Dumbledore!
Snape: You know perfectly well that I do not wish to do either of those things.
Draco: You'd better stop telling me to come to your office then. People might get the wrong idea.
Snape: You've been spending far too much time with that Harkness boy.
Owen's methods of psychological torture.
The first half of Chapter 94. Particularly the Polyjuice section and Jack's smirking.
The Will scene:
Scrimgeour: But a Snitch would be a very good hiding place for a small object. You know why, I'm sure?
Hermione: Can't imagine why. They're not hollow, are they? How would you even open one?
Jack: *brightly* I can think of better places to hide small objects.
Ron: No. No no no no no!
Jack (on seeing Ron play with the deluminator).
I want a lightsaber.
Most of chapter 99.
Jack casually walking into the safehouse with cinnamon buns.
Then Lucius makes the mistake of mentioning that Bellatrix was given something that she referred to as 'precious'. Jack promptly goes to town with the references.
Jack: Right... we need to find out where Bella's hiding her precious.
His suggestion to train a house elf to talk like Sméagol.
Harry's POV: He only noticed Tosh turn and whisper something to Owen, because Owen was a dangerous creature you don't take your eyes off if you know what's good for you.
Harry as a woman, trying not to stare at his own breasts.