The second my players heard my Giorgio voice, and his request, they immediately stated (out of character): "It's a trap. He's going to kill everyone."
Without even excitement in their tone.. they seemed almost disappointed in how obvious it was.
So, after they made their attempt at swaying the leaders to not do this, and the response being that they "had" to, in hopes that it was put down some of the rising anger each group is having for each other, I quickly winged it.
I had the players sit with Simeon, and gave them this little speach:
"Listen, I know as much as you do, that if there ever was going to be a moment where an assassination attempt is to be made, it'll be at this play. But here's the thing: knowing it's a trap, we can be ready for when they spring it. Catch them in the act and prove that everyone is actually in danger, not just the Lyceum, and perhaps gain some leverage.
I know it's risky, but with the others determined in participating, we are pretty much stuck.
I will need you there, not just because you've proven yourself very capable in combat, but also because with all the attempts on your life already, you are among the very few people I can potentially trust here.
We will go into this trap with our eyes open, and with our guard up. I hope my trust in you isn't misplaced."
That really put them in the mood to participate.
Pretty much none of the players were into the play itself (they don't like that kind of thing), so while they were making checks to see if anything suspicious was going on, I only gave them a thumbnail description of the flashier bits of the play.
Katrina was there, acting fairly non-plused at the event, making the occasional comment on how the play was going (overacting here, trying too hard there, etc) that the party was pretty much ignoring. They seemed bound and determined to dislike her for some reason (too much attitude?).
When Giorgio came out his hiding spot wiping blood from his hands, half-waving at them nonchalantly, the players didn't think anything of it (he had died on stage), but I had Katrina yell at them that there wasn't any blood in his death scene... initiating the actual chase/combat from that point.
It turned out pretty good. Despite guessing almost the entire course of events, they still felt driven to participate and ended up still having a surprise moment as well.