Anytime Misaki freaks out, or lays on the ground in defeat.
Especially the scene in which Usagi tells Takahiro that Misaki wants to live with him (rather than moving away to live with his brother). He basically makes Misaki out to be the most overly dramatic uke cliche possible without actually revealing that Usagi and Misaki are together. Meanwhile, in another room, Misaki can hear everything he's say, and falls to the floor- and when Takahiro overhears the THUD, Usagi claims he's looking after a neighbor's Pomeranian.
[Reading on of Usagi's BL novels has homework]
Misaki: Oh, pssht! So this is what Usagi wants me to do? This is all? This is it? Ha ha ha! I'm in college now, of course I... WOULD NEVER FUCKING DO THIS!!!![launches the book out the window]
Usagi: My housemate hasn't spoken a word to me for a week. What could be the problem?
Hiroki: Whatever it is, I know it's 100% your fault.
Miyagi starts asking Hiroki about a hypothetical kid who might have a hypothetical obsession with him. Hiroki plays along until Miyagi admits this hypothetical kid is a guy. Hiroki thinks he's talking about him and Nowaki:
Hiroki: What has this whole thing been about? If you've got a problem with me, why don't you say it to my face! What would you know, professor?!