An “excerpt” folder.
Every writer has one. There’s scenes or bits of dialogue that are so loud in your head, you have to jot them down. They might not fit into what you’re writing right now, but it’s too good to let slip away.
I have dozens of these files, and oftentimes, I end up not using them.
However, they are important to the writing process. Why?
Because they tell me details I otherwise would’ve never learned.
You can discover a lot about your character in the smallest bursts of dialogue or action. Make a habit of writing down EVERYTHING.