Victor Garber guest stars in this episode. Both he and Len Cariou, who plays Henry Reagan, were in the original cast of Stephen Sondheim's musical Sweeney Todd. Cariou played Sweeney while Garber played Anthony Hope, the romantic lead. Both can be heard on the original cast recording.
Victor Garber plays Donald Stein. Stein would be the surname of the character he plays in the DC Universe shows The Flash and Legends of Tomorrow. That character is Martin Stein aka half of Firestorm.