Start with the original Broadway cast
The original cast album gives you an audio record of the first Broadway cast and a way to hear the score as a sequence. It is also an album made for listening at home. Use the Library of Congress essay to place it in context and consult track listings before assuming that it preserves every repeat or transition heard in the theatre.
Then hear the two film soundtracks
Watch or listen to the same two songs in the 1961 and 2021 versions. Make a three-column list covering performer, location in the plot and what the accompaniment is doing. Keep a fourth note for ensemble or instrumental material before and after the vocal. Changing the space around a number can alter its effect as much as a changed singer.
Try the “Tonight” test
Play the duet, then jump to the ensemble reprise. Make a rough listening map of when each group enters. Notice when a musical line you already recognise becomes the background to someone else's anticipation. You can use the same exercise with an audio recording and an on-screen performance to see how framing redirects your ear.