Pre-prise?

I was watching the movie Molly Brown on TCM today and I noticed that
characters were singing little snippets of Belly Up to the Bar, Boys
before Molly actually sings the song through (apparently to establish
that she knows the song from somewhere),

I also remember that in Cats, Grizzabella sings a shorter version of
Memory at the end of Act One, then sings the entire song in Act Two.

Think of any others?

(There are some shows like Wonderful Town! or the Charlap, et al Peter
Pan that introduce the music in some other way, then use the music in
a song: I don't mean those, but they are a similar case.)