A brief moment of realization that I think some people miss during 'Don't You Forget'
When Al asks Rosie to fix his staff, she says 'in due time. sing along dear, you know the words'.
So not only is this song diegetic (it exists within the world of the story, basically rather than being a stylized musical number, they actually are singing to each other), they've done this before.
u/ChompyRiley — 16 hours ago