This is the very first scene where the salesmen are on the train discussing Harold Hill and how he "doesn't know the territory!"
"Marcellus" is on the left and "Harold Hill" is on the right. "Marcellus" is telling "Harold" about "Marian" during this scene.
Sam and one of his new friends waiting for one of their scenes. Okay...doesn't his friend look like a young Ron Howard???
Here's two quick videos of the teens, Harold, Marian dancing during "Marian the Librarian". They had only rehearsed this dance for maybe an hour or so before hand.