Watch the Riverside: Rhythm 2 online guitar lesson by Andy Aledort from Jam Night Vol. 1
Part of the idea with this secondary rhythm part is to provide an improvisational feel to what is essentially a basic rhythm part, and this is accomplished via subtle changes in the chords and surrounding riffs as the part progresses. I begin with a higher voicing of Em9 and use voice-leading in the change to A7 as each of the notes in the chord move down a single scale degree within the E Dorian mode (E F# G A B C# D). I also use E Dorian as the basis for the single-note riffs, as the major sixth of E, C#, serves also as the major third of A. The Em9 voicing is then expanded to Em9/11 as the index finger barre includes a high A, first string/fifth fret. Once you have this part down, try changing around some of the notes in each chord voicing as well as experimenting with different riffs between each chord.