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.
She completely losses it when George takes off his wig, prompting him to step on it a few times to show her its safe.
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.
George's father trying to find a place to punish him, each one being shot down by his mother due to theater necessities such as dancing and singing. Finally, he settles for a good old-fashioned spanking of George over his knee, since it's the "one part without any talent".