To what Esthete said, all performance is acting. That's why we call it "performance."
Well, not quite, but this idea that you are seeing a real person up there is pretty bogus. Sincerity is a sham. You are seeing a stage persona. You are seeing what the performer lets you see.
Joe Cocker, in other words, was almost certainly faking it as much as the next guy.
All that really separates that oh-so-sincere performer from the poseur is acting ability. A good friend of mine disputes this, saying that you can't hide, that as soon as you step up there your personality is on display and the audience can see right into the centre of your soul.
And to that, I always say: Bob Dylan, 1963. I been up, I been down, I been all over these here United States and I blew into New York on the wind wearin' my hard-travellin' shoes and playin' my guitar and a-singin' in my hard-travellin' Okie voice and I seen all these places and things in this funny old world ta sing about. He was the son of a shopkeeper from Minnesota, and had hardly been away from home. But everyone bought that persona. Not because it was real. Because it was convincing.
The only thing that makes my friend right is that many people don't bother acting; they just put their personality on display in place of a constructed persona. That audience still only shows what they choose to display.
So yer objection to Hicks isn't that he's acting, so much as it's that he's not convincing.