I've played Dead songs my whole life, but just recently hooked up with an actual Dead cover band for the first time (called Mason's Children). It's been a blast by the way. The other guitarist and I trade off parts depending on the song, so I get the "Bobby" role on many tunes. A few observations so far:
1. Bobby's playing is unlike that of ANY other guitarist I've ever heard, and contributes a great deal (probably as much as Jerry and/or Phil) to that overall intangible quality of the Dead's sound that is very hard to mimic.
2. I find myself having to listen VERY intently to the rest of the band when I'm in the "Bobby" role, to make sure my chords and rhythms fit in without stepping on anything else. I find myself having to listen even more intently than I do when I'm in the "Jerry" role--when I'm "Jerry" I can just sort of float over everything, but when I'm "Bobby" I feel like it's more important to support the sound of the whole band, which requires intense listening.
3. Standard bar-chord voicings often sound way too dense within the Dead-y context. In many cases, the simpler the better, such as 2-, 3-, and 4-note chords.
4. I find that it sounds good to replace regular major chords with "sus2," "sus4," and "add9" chords. On minor chords, I often substitue "m7," "m9," and "m11" chords.
5. The rhythms within Bobby's playing are super-important as well (a topic that hasn't really been discussed in this thread yet). They often form a counterpoint to other rhythmic things that are happening within the music (this goes back to the listening thing). A great example of this kind of thing is his part in "Scarlet Begonias."
6. In general, I think of the "Bobby" role as having to do with things like color and texture, as opposed to the "Jerry" role of dripping icing all over the cake.
7. I also find that, as "Bobby", my playing can drive the overall dynamics of the music to a great extent. In a jam, if I pump up the energy by playing louder or more rhythmically "busy," it forces the rest of the band to step it up too. Again this goes back to listening, and must be done tastefully, but it totally works.
I could probably add to this list if I sat around and thought about it, but these things jump to mind immediately.
And in regards to Rob Eaton from Dark Star Orchestra (who I've met several times and is a super-nice guy), I have gained a whole new respect for what he does. What amazes me in particular is that he can do "70's Bobby" or "80's Bobby" or even "90's Bobby", mimicking not only the playing style, but the guitar tones as well. I may go ahead and shoot him an e-mail about this--if I get a reply I'll post it here.
Great discussion, folks...I hope we can continue it!
Paul