Dawson Casting: While throughout the film Rath is treated as an old man and Bain as a young punk, the ages of the actors don't particularly help: Stallone was 49 years old, Banderas 35-years old.
Hey It's That Cat!: A certain feline cosies up to a hardened, but code abiding, assassin yet again.
What Could Have Been: The original casting ideas called for the Rath role going to Sean Connery, Michael Douglas and/or Arnold Schwarzenegger. (At the time Connery was in his 60s, Douglas in 50s, Schwarzenegger in his 40s). Ideas for casting Bain included Tom Cruise, Woody Harrelson and Christian Slater. (Again, at the time Cruise and Harrelson were in their early 30s, Slater in his 20s).
Acting for Two: In the 2004 revival, the Balladeer and Lee Harvey Oswald were played by the same actor. Many productions since have followed suit.
Hey, It's That Guy! / Hey, It's That Voice! (depending on whether you watch or listen to the show): The original production featured Anthony Hope as Booth and Inspector Javert as Czolgosz, while the Broadway revival featured Tommy and Edward Rochester, respectively in those roles.
Of course, possibly the revival's biggest star was Doogie Howser / Dr. Horrible as the Balladeer and Oswald.
Who, a few years earlier in 2001, played Toby Ragg, also from Sweeney Todd, in the concert version. I'm seeing a pattern here...
The revival also featured Lindsay Weir's mom as Sara Jane Moore.
Funnily enough, in contrast to Anthony Hope as Booth in the OBC, the revival cast as Michael Cerveris as Booth, who later went on to play... Sweeney Todd in the most recent revival.