Some of the most admired aspects of Robert Johnson's guitar work were his intros and "turnarounds" (the lick at the end of a song that leads back to the beginning). The tab presented above is one of Robert Johnson's intros to a 12-bar blues in the key of A, in both standard notation and tab (listen to an mp3 of the lick above).
This is a blues intro that, with slight adjustments, will work in many different situations. Playing the intro in different keys will require playing a barre chord in the fourth bar of the lick, instead of relying on open strings.
If you're going to try and play the above with a pick... you're going to have a more difficult time. Robert Johnson, in keeping with many blues players of the time, played without a pick. Playing without a pick enabled Johnson to include moving bass-lines, while also playing upper register notes that moved independently of the bass figure.
