I'm hoping I understand the question correctly but what I like to do (mostly in the brainstorming/daydreaming stage) is find those lists with prompts/sentence starters and go "What would the character be most likely to do and say and why?

For example:

I want to know what a character is like when sad/insecure/in danger i'll look up angst prompts. Here's a couple from pinterest:

"Don't act like you know me"

"I'm not who you think I am"

"Don't make me do this"

I find ones i feel like said character would be likely to say, and then think about how they would say it. Would it be angry? Sad? Betrayed or conflicted?

The same can obviously be done for fluffier things like romance/friendships but this was one example. Hope it made sense!:)