Watch the '61 SGLP: A Section online guitar lesson by John Jorgenson from Eclectic Electric: 61 SGLP
You just heard the A section - now let's break it down. The first phrase is really built around chords - E, G, D, and A. Basically, you play one note on each chord. When you get to A, you want to grab the G on the E string and rock back and forth (with some vibrato).
If you notice, my right hand is doing a few things that you wouldn't necessarily notice to keep it clean. By muting, it allows me to play the chords "short", and gives me more clarity and punch in the riff. So, in between the chord, I'm muting all the strings with my palm, and muting just a little bit when I rock back and forth between the A and the G. It's especially important when you have distortion on to mute the ones you don't want ringing and to keep it clean and punchy. Distortion adds a lot of sustain and overtones, which is fantastic, but it doesn't necessarily know which notes you want to sustain.