I'm listening to the Horace Slughorn chapter in HBP right now and I'm reminded why i love and am honestly really intrigued by the relationship between dumbledore and slughorn
Bc Slughorn sees Harry and he instantly clocks that Albus is trying to manipulate him. (The exact phrasing is: Slughorn turned on Dumbledore, his expression shrewd. “So that’s how you thought you’d persuade me, is it? Well, the answer’s no, Albus.”) And then he goes and let's him! He might grumble about it and play hard to get for about half an hour but once he says yes, he knows exactly how they got there.
And in general, Slughorn is the character who most treats Albus like a peer. There's real familiarity in the way they talk. Slughorn comments on Albus' age and injury from a perspective of someone who shares in this experience when most people will either not comment on it out of respect or else, comment on it in a way that implies that Albus is loosing his touch.
And obviously Albus knows Slughorn, too, or he wouldn't have so effectively manipulated him in the first. Only, Albus also knows that Slughorn is in the know.
Albus: here is Harry Potter, the current most famous student at hogwarts, wouldn't you like to collect him into your network?
Slughorn: i know what you're doing old man and I'm not falling for it
Albus: well fellow old man, I actually think you are falling for it.
Slughorn: ... this is like in 1932 when you got me to take your detention supervision duties for a months. Alright alright alright, I'll come back to hogwarts!
I'm just finding myself endlessly fascinated by the potential of this dynamic: two intelligent, manipulative and intensely isolated old men who have known each other for decades and slowly watched their generation disappear, one of them approaching his death ...