From what I've seen, interference works a couple of ways. First, you can select to interfere in any given match, and you get to choose who you want to use to interfere.
When the AI interferes, it's random from what I can tell. I've seen R-Truth come out to help Miz early in a match, but I've seen ADR come to the aid of Christian after 4-5 minutes. Like I said in an earlier post though, the interference rarely involves a guy actually getting in the ring.
As far as cutscenes, I've seen guys run down the ramp and come out of the crowd. Don't know if there are more than those two.
To be honest, though, I've been doing more simming than playing, so I haven't actually seen that many cutscenes in Universe. In fact, I wish there was a way to reset Universe because I think I've missed most of the cool moments by simming through them. I never see any of those remarkable moments that were so frequent in the first couple of months.