About 20 years ago I was backing up a Melbourne singer, Sharon Anderson.
I didn't know the songs, so I just copied her vocal lines.
It worked well, and gave me an insight on how "vocal" the
slide is.
My mates and I used to copy people talking, for laughs. Just plain speech, and phrasing. It was fun at the time, but with retrospect it was probably good training.
I've copied trains, chickens, seagulls, sirens, drums, all sorts of things.
The slide is pretty cool for emulating sounds.