I once wrote a novel about a young American man in West Berlin falling in love with a young Russian man.
Before I started writing I constructed a detailed family history for both of them. I knew that would be important because it would shape how they would interact with each other coming from different systems of values.
I referred back to it many times, and by the third Act, it turned out that the Russian man’s family became very important players in the story.
I hadn’t intended that when I first imagined the novel, but the character work I had done beforehand made implementing it really easy and natural.
It also meant I didn’t have to go back and rewrite a bunch of stuff to make it match the third act.
It already fit, because I’d created and peopled my universe before writing.