To get the full scoop, let's look real quick at a couple of Robert's intros and turnarounds. For a song intro, over half the time he used some variation of the intro shown in bars 1-4 below (this one is taken from "32-20 Blues"). The first 2 bars of his intros tend to vary, but often utilize a progression moving down the neck from this A7 fingering:
Bars 3-4 have a simple trick; you hit the first string on the 5th fret repeatedly, while moving down the 4th string from the 5th fret to the 2nd fret. This leads into a few quick notes played out of an E7 chord fingering, which then leads you into your song. Every time you hear this type of intro from Robert, it will sound slightly different, but the main difference is in the timing of what he's playing, not in the actual notes being played.