What NPC interactions are there? So far I have seen:
Aerie/Haer'dalis: If you reject Aerie eventually Haer'dalis starts coaching her in acting and they fall in love.
Aerie/Kelgorm: Aerie seems to regard him as some sort of mentor.
Aerie/Jaheira: Catfight...at least at the start.
Aerie/Minsc: Aerie become Minsc's new witch.
Minsc/Edwin: Hatred
Minsc/Kelgorm: Respect -- I can almost see Kelgorm rolling his eyes though.
I know this is just the tip of the iceberg.
NPC Interactions
There's a funny one: Minsc and Jan Jansen. Jan will try to steal Boo from Minsc, and if I'm right, eventually succeeds - much to the anger of Minsc of course. Whether this leads to armed conflict I don't know, but this is something funny ... my brother currently has them both in his party, to see what happens.