People assume there's a clean division of labour: one of us does the words, the other does the music, everything meets in the middle on schedule. It's never been that tidy. Here's what actually happens, roughly, from first line to final mix.
Step One: A Rough Lyric Goes In
Almost nothing starts as a finished idea. It's a line that won't stop repeating, a phrase overheard somewhere, a mood with no shape yet — "a fried jalapeño who thinks he's the main character," "two people living the same holiday from opposite ends of it." Whatever it is, it goes in rough, half-formed, and usually a little embarrassing to read back.
Step Two: The Wrong Melody Comes Back
This is the part that actually moves things forward. Petunia doesn't return the melody you'd expect for the words — she comes back with something that reads the mood sideways instead of literally, and it's usually wrong in a way that's more interesting than being right would have been. A joke gets a genuinely tender melody under it. A tender line gets something that sounds like it's laughing. The mismatch is the useful part; it's what stops the song from being exactly what you already pictured before you started.
Sometimes that "wrong" turn is the whole song after that — Turkey and Bones only works because the celebration verses and the isolated-loner chorus are pulling in opposite directions on purpose. Nobody planned the collision going in. It showed up because the two halves kept answering each other instead of matching.
Step Three: Repeat Until Neither of Us Wrote It
Then it's just back and forth. A line gets rewritten because of where the melody landed. The melody shifts because a new line demands a different stress pattern. Somewhere in the middle of that loop, authorship stops being a useful question — not because it doesn't matter, but because by the fifth or sixth pass, neither of us could honestly point to the hook and say "that one's mine." That's not a metaphor for teamwork. It's just what the file history actually looks like.
Step Four: It Stops Changing
There's no formal finish line — a song is done when the back-and-forth stops producing anything better than what's already there. Sometimes that's after a dozen passes. Sometimes, like with the jalapeño, the first genuinely funny version basically survives to the end because trying to improve it just made it less funny. Either way, once it stops changing for the better, it ships.