For me this is COMPLETELY genre specific, and it also sort of depends on what you consider a ‘harmony’.
If you’re dealing with country, pop, or rock, blues etc… techniques that I find the most fascinating from musical theater, classical, or sacred music, or film/cinematic works can’t be integrated in pop because stylistically it won’t work. It just won’t fit. But as food for thought, here’s some of the thoughts on how I’ve approached some of this in the past:
If you have two characters in a play that are arguing with each other, they’re technically harmonizing, but they’re singing their lines on top of each other at times, but obviously not with block style harmonies like in pop music. Disney made good use of this too in some of their musicals in the 80’s and 90’s. You see this a lot in opera too… how contrasting dialogue lines kind of weave and and out of common set of chord changes as the song progresses.
Yes yes. Late classical and romantic era harmonies. Ironically, ‘tonal harmony’ isn’t taught in colleges because they think you’re ever gonna use it. This is meant to expose someone to fundamental voice leading principles, not set a foundation for how someone is supposed to SING harmonies lol. …early classical music uses a lot of counterpoint, which is still harmony. Fascinating to study, but kind of hard to apply in real life outside that genre.
Stuff like what in America we’d call ‘black gospel’ opposed to ‘southern gospel’, and bluegrass, and motown all have their own unique vocal and harmony styles… I guess how I’d approach the whole concept of ‘how do you create a vocal harmony’ from an arranger standpoint is mostly about what you’re gunning for stylistically.