Interactions

I personally didn't find chapters 19 and 20 to have that much about the people he's met, the only person he really talked to and could get something from was Luce. But looking through the whole book and thinking about the characters in those chapters you can see much more.

Allie may be one of the most influential characters in the book, at least to Holden. Every time he speaks about his little brother there is so much love, and you see the shock and pain he went through that he did himself did not understand. His little brother was purity and innocence embodied (in Holden's mind) which was probably why Holden hadn't let his brother come along with him and Bobby when they were going to go and shoot stuff with their BB guns. So when he had originally denied his brother from coming it may have been to preserve his person, but obviously it is one of the greatest things Holden regrets, perhaps that was one of the last interactions he had with his brother.
In chapter 20 he talked about Allie's funeral, how he was disgusted by all these relatives that probably didn't really care to much about his brother. And then the cemetery, you almost got this feeling that death petrafied him, what they did when you were dead. Allie, to Holden, seemed to be mistreated by being stuck int he ground around a bunch of dead guys that probably never knew this kid that joined then in the ground. And then putting flowers on him when there seemed to be no use of them for Allie. And then the rain, what are they doing to us, just putting us in the ground so that it rains on us all the time?
You see him remembering his little brother, talking about him when the situation arises. He cares about Allie even now and clearly misses him. And I think finding conversations, and reading literature brings him closer to his late brother. I suppose that's the interactions I've seen