I laughed my ass off when Mary first meet George and freaked out when he pulled of his fake beard (he'd just been playing an old man). Mary of course had assumed he was the senior Cohan, and screamed when the beard came off.
Dietz and Goff.
George and his competitor Eddie Foy's friendly banter back and forth after Foy readies the sign for the next Cohan show.
One of the best scenes has George and Mary's World Tour.
Mary: George, I didn't know you could yodel.
Cohan: Learned it on the farm. Nothing but pig callin' with frost on it.
According to some inside information, the scene where Cagney goes into a tap dance while walking down the stairs at the White House was completely ad-libbed.