I have a student working on becoming an arranger, and we’ve been meeting occasionally to review his writing.
On our last meeting, I recommended he sing through all the other parts of every piece of music in his choir folder. It’s not enough to know the theory and build chords that work – you need to know how an alto part feels. His didn’t feel right.
Meanwhile, my dad is being challenged singing baritone in a divisi section of a Rutter piece. He’s gotten used to singing bass in his community choir, but baritone parts don’t feel like bass parts. (They feel more like tenor parts)
Write an alto part that doesn’t feel like an alto part, and the singers will never feel fully comfortable with your piece.
Learn to write what you want to hear in a way that’s intuitive to your singers, and they will always want more of your writing.
You have to learn to feel the baritone part.