When I’m developing a story, I’ve found it helpful to record “conversations” between characters. For some sections of the story, I can imagine interactions between different characters, sometimes quite vividly. I then act out their dialog, and if it’s convenient, record it using my phone or another device. Sometimes, I’ll write it down, but I’ve found that this tends to slow down the easy flow of the “conversation” my characters are having.
A little weird, I know, but it works for me and helps me to get to know them better. I may not actually end up using the conversation in the final narrative, but it definitely helps with story development.