Two decades late, but inspired by the movie of all things. So here's a brief analysis of Wicked Act I and the importance of key signatures.
First an overview of post-Wagner key signature conventions.
It is quite common post-Wagner to have the entirety of an act or even the whole opera to work towards the resolution of one chord. Composers sometimes are very careful to never play the final chord in order to trigger that feeling of catharsis and arrival. Wagner was known to underpin every song with the dominant, while also orchestrating it so it feels unstable, thus making the audience on edge. So once the final chord is played, there is a feel of "finally". He weaponised music theory to make the audience feel just as much longing as the character themselves. Schwartz is well-versed in music history, so I have no doubt the following key signature choices are intentional. This is especially true since many well-known composers also utilised this technique, such as Sondheim, Webber, and Zimmer. Schwartz also collaborated with Zimmer multiple times, and we can see their influence upon each other in later works.
Now, let's break down the keys of the major musical numbers we'll be looking at.
1) The Wizard and I - Ab+
2) What is this Feeling & Popular - C+
3) Dancing through Life - F+
4) I'm not that Girl - A+
It's clear based on orchestration and important moments that Schwartz had chosen to represent characters and their "true selves" using a key that represents them. Elphaba is Db+ because of its darkness and seriousness. Galinda's is C because of its purity and cleanness. Fiyero's is F for whatever the fuck. It's like the hardest key to sing in but go off.
Now, the relationship between "The Wizard and I" and "Defying Gravity" is meant to demonstrate that the "I want" song is the dominant to the resolution of Elphaba's character arc in "Defying Gravity". This is why "The Wizard and I" cycles through a bunch of keys while never touching Db, each time climbing higher and higher, closer and closer to Db, but stopping short at C.
Galinda being in C+ demonstrates how close she is to Elphaba, but the two actually being very different. Db and C are not pleasant notes to hear next to each other. But, C is the leading tone to Db, so in a way, Galinda is a very necessary part of Elphaba. She is the stepping stone to Elphaba becoming who she really is.
The most interesting part for me is the keys between "Dancing Through Life" and "I'm not that Girl".
Fiyero's key is F+, this the audience and Elphaba knows because we heard his song before Elphaba's final transformation. Elphaba likes him but doesn't think she's the right match for him. This realisation is expressed through "I'm not that Girl", which is in A+. A+ is the mediant of F+, expressing a desire to harmonise with him and be a pair with him. It is one of the rare songs that doesn't skip through a million keys, demonstrating her very earnest desire to be that girl, but recognising she is not.
However, with "Defying Gravity", it becomes clear that Elphaba's key is Db major, which means Fiyero is the mediant to her key. He is meant to harmonise with her. Elphaba is so desperate to spin around someone else's orbit, that she doesn't realise she's literally the sun and Fiyero spins around her.
We see a glimmer of her realising this is true in "I'm not that Girl" as well. In "I'm not that Girl", Schwartz uses compound time to represent fantasy, and simple time represents reality. When she's talking about "what could have been" in her 6/8 fantasy land, the chords are in Db major. But only for two fucking bars. Schwartz goes Ab -> Db (cadences) but it's Db add9, meaning it doesn't quite sound right. However, it is an early hint that Elphaba recognises this as "a love she assumed she lost" when in reality, she knows that she is the tonic to her own song.
Also, at the end of "Popular", Galinda modulates from C+ to F+, demonstrating that being popular ultimately is about being like Fiyero. It's also reflected in the lyrics of both "Dancing through Life" and "Popular", where the underlying ethos is "form over content". So at that moment, we truly feel like Galinda and Fiyero belong together due to their shared ethos and now shared key.
But Elphaba changes everything... Because with the last song, we recognise that as the true cadence. The entirety of the first act is in Db major, making Ab the dominant (the earnest desire), F the mediant (a pretty harmony), and A an unfortunate passing tone. Elphaba was never meant to be someone's girl. Fiyero was meant to be her guy.
