"Soul" presents many of the same problems as the concept of Realism / Naturalism in fiction or acting (for film or stage)
You have a breakthrough into what seems - to the audience who embraces it avidly - an unprecedented rawness of emotion, a directness of feeling, no barriers, no prettification - the Real of desire in all its violence and shattering intensity.
And actually before soul and R&B takes on that role, the very first example of this in UK postwar pop music is skiffle - Lonnie Donegan singing the songs of Leadbelly. His wildfire success is through introducing into bland Fifties pop, what feels raw and real. 1000s of groups spring up making skiffle overnight, including the future Beatles.
This parallels what's going on in the theatre and fiction - Look Back in Anger (the ironing board in the living room, the husband ranting away), angry young men, Northern realist novels like Billy Liar, the kitchen sink social-realism movies.
But when you look back, it is blindingly obvious that Lonnie Donegan - for all the commitment he throws into his performance - is staging a performance, a pretence to something he's not (how could it not be, his experience is a million miles from Leadbelly), something just as stylized as Sinatra or Alma Cogan
likewise all those then innovative and shockingly naturalistic styles of acting (cf. the Method in America), as time go by reveal themselves to be just as mannered and performed as the earlier Lawrence Olivier "speaking your lines very loudly and clearly so the back of the hall can hear them" style of English stage acting.
or James Dean - i remember going to see Rebel without a Cause in the 80s, in a cinema, having since once before on TV and loved it - but almost as soon as it started, some in the audience started giggling, including me and my friends - cos his mumbling and punching the policeman's desk etc seemed almost camp - but others, older - who'd grown up with the movie when it came out, been liberated by it - turned around to hiss at us, for the sacrilege.
And the same goes with soul
- it hits, initially, as the hardest hit of realness yet - the raw grain of the timbre, the cries of passion, no politeness, no restraint, pure feeling from the depths
but Aretha, for one, rehearsed every last shriek and gasp
black soul and R&B etc in its own way is just as showbiz and performed as any Las Vegas routine
Janis Joplin, who believed in the mythology of soul with every fibre of her being and desperately wanted to be black, likewise planned out her vocal performances in the studio very carefully - they are not spontaneous eruptions of emotion taking her over, she's very much not "out of control"
so Soul - quickly became, or perhaps always alreadywas - Technique, and as such, capable of being learned and copied by people who didn't have the authenticating life experience (being black in America, basically).... a formalized language of vocal gesture and matching physical gestures that could be reproduced with absolute hollowness
what seems like Interior Truth is revealed to be Exterior Technique
So basically Soul / Realism in singing and acting needs to be reinvented, as each new mode of expression become a set of cliches - becomes in fact soul-less / un-Real