How we cite our quotes: All quotations are from The Departed.
Quote #4
QUEENAN: For Chrissake, be smart. If anybody's watching us now, how are we not supposed to arrest you?
Everything gets crazy when Billy punches Dignam in the face during an argument. Queenan tries to calm things down by reminding Billy that his cover would totally be blown if anyone saw him punch a cop and not get arrested. Looks like he's the only one actually being smart at this point—which makes sense when you consider how much stress Billy is under.
Quote #5
QUEENAN: I think Costello's got a spy inside the Special Investigation Unit.
Queenan is no fool, and he's smart enough to realize that Costello must have someone inside Special Investigations. There's just too much that the dude knows for him not to have someone on the inside.
Quote #6
BILLY: Don't tell anyone in our division, but tell SIU. Flush it down the pipe; see if it comes out on my end. That's what we do first. Narrow it down.
When Sergeant Dignam himself has no ideas on how to catch Costello's mole, it's up to the clever Billy to come up with an idea for how to do it. Billy starts feeding Dignam very specific info and tells him to relay it to certain people in the department. Then Billy will listen to see if anyone on Costello's end starts talking about it. If they do, he'll be able to narrow down who could have possibly passed on the info, which is pretty clever of him.