Well, it's more than that. And Booth does have the biggest role of the assassins.
The assassins interact with each other in a limbo state (obviously, as they are from different time periods). Breaking up their conversations is each individual assassin's story in which you learn of their motives and how the attempt went down.
There is a Balladeer that sort of cheerfully narrates the tales. Booth has his own segment and then sort of acts as a constant presence pushing the others to carry through with their attempts.
I definitely suggest buying the cast recording and see it if you ever get the chance!
It's brilliant and very thought-provoking...albeit, hard to explain.
"My friends have made the story of my life."
-Helen Keller