At the end of "All Good Things," when Q appeared onboard the Enterprise and spoke to the old version of Picard, do you think Riker and the rest of the crew could see him?
I think it was more a matter of Q purposely being visible only to Picard. All those timelines were reset anyway, except from Picard's mind, so it's moot anyway.
I'm not sure if Q ever appeared selectively to only one person in a filled room, which could establish a precedent. And since he created the temporal paradox in the first place, the whole thing could have been some sort of illusion. Though that would weaken the impact of the story.
Call me crazy, but I don't remember a scene like that anywhere in the episode. Not where Picard and Q were on the Enterprise talking in a position where others could see them.
It was at the end, when the Enterprise was trying to close the rift. Q appeared next to Picard, and then threw out his famous line: Goodbye, Jean-Luc. I'm going to miss you. But then again, all good things must come to an end. Or something to that effect.
I originally thought the thread title was referring to this: [yt]http://www.youtube.com/watch?v=VBTRp80Q64U[/yt]
The amount of control he had for stay exactly in place while someone put a cigar in his mouth for the effects edit. It's not easy to do.