There are actually several G3 performances that you can't get on DVD/VHS, I believe, and I think that the Robert Fripp is one of them. The ones I know of that are available are the one with Eric Johnstone, G3 '03, G3'05, and '04, I think.
As for Petrucci's lack of theatrics...I'll just copy/paste my rant from another forum,

To me, he just comes across as a shyer person than Satriani or Vai (I'm not even going to bother mentioning Malmsteen....) In the commentary on the soundcheck on the G3 Live in Tokyo DVD, there were often long pauses on his commentary track, and he seemed to be a quieter person than the other two, who just wouldn't shut up. Another thing that seemed to show that he was a quieter person was that when Steve hauled out the rubber chicken at the end of the soundcheck (it's worth buying the DVD just for that scene) John laughed and made comments, but they seemed to be much more reserved than what he was actually thinking.
I think that he is more of an introverted person, and that that comes through in his stage antics, or lack thereof.
EDIT: But hey, I'm an actor, and analysing people is part of my job. I might just be blowing things up a bit.
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In my experience, people that are quieter in social situations are almost always less exuberant and big onstage. In fact, after two acting tours, three acting workshops, and the first four months of a high school Performing and Visual Arts course in drama, I can think of one person who was socially quiet and big onstage.
And since Mike Portnoy is his drummer, I would think (or at least hope) that he was comfortable enough with him to talk into a microphone. But on the spot, all alone, he seemed to be much quieter. And that makes sense, because if you think about it, there's no one to support or to fill dead spots in the commentary.
This is, however, not to say that he doesn't enjoy being onstage. On the G3 DVD, especially on the jam, he was smiling a lot and obviously having a great time. However, I think that this proves what I said above about him being more relaxed when he was someone else to be with. During most of his solo performance, when he looked at the audience, he looked at what would have been a spot above the heads of the people all the way up in the mezzanine. I think that he was trying his utmost to not let the audience intimidate him. And by all means, he did an absolutely incredible job. Glasgow Kiss and Damage Control were flawless.
During the jam, he was much more "outgoing," and appeared to be much more relaxed, which was in my opinion because Steve and Joe were up there as well.
...and that concludes bucky's second installment of the "Let's Overanalyse a Performer" series.